This is a highly complex topic which can arouse strong views. I have attempted to tackle some common issues, but my main point is that ease of reading should take priority over rules.
a/an: It used to be correct to use ‘an’ before an originally French word beginning with ‘h’ (which is silent in French) such as ‘hotel’. It was also considered correct to use ‘an’ before a word beginning with ‘h’ where the stress is on the second syllable, such as ‘horrific’. I think both rules are impossibly old-fashioned and should be discarded. Use ‘an’ in front of a set of initials which when spoken aloud start with a vowel, such as ‘an MP3 player’, or words beginning with a silent ‘h’ such as ‘an hour’ or ‘an honest man’. Otherwise if a word begins with an audible ‘h’ use ‘a’, as in ‘a hotel’ or ‘a horrific accident’.
active and passive voice: Should not be confused. For example, ‘he shears/sheared the sheep’, but ‘the sheep was shorn’; ‘he mows/mowed the lawn’, but ‘the lawn has been mown’; ‘the torpedo sank the ship’, but ‘the ship was sunk by a torpedo’. It is a common error to say ‘the torpedo sunk the ship’. See past tense and past participle, below.
adjectives and adverbs: Use sparingly or they can make the copy slow and tedious. Descriptive words are fine for novels, less so for journalism. You do not need to hyphenate adverbial compounds where the adverb ends in ‘-ly’, such as 'the stylishly dressed man'. Use a hyphen only when the adverb does not end in ‘-ly’, such as ‘the well-dressed man.’ When the same form of words appears after the noun, do not hyphenate. For example ‘the man was well dressed’.
gerund: A gerund is derived from a verb but acts as a noun to name activities, states and behaviours, for example ‘swimming is easy’ and ‘he knew that stealing was wrong’. All gerunds end in ‘-ing’. For our purposes, the main thing to know is that it takes a possessive if it follows a noun or pronoun, for example ‘John’s asking him to be best man left him speechless’ and ‘What do you think about his buying such an expensive car?’
hanging or dangling participle: An example of the construction would be ‘Born in 1926, Princess Elizabeth went on to become Queen’. The first (dangling) clause must always refer to the first person (or object) mentioned in the second. If you do not observe this rule, you can get ridiculous muddles. A great example came from BBC radio in September 2009: ‘Fifty years after his suicide, Prime Minister Gordon Brown has apologised to computer pioneer Alan Turing.’ I noted a particularly grotesque case in the Daily Mail recently: ‘Dressed in a Muslim veil and surrounded from all angles by armed officers, this was the astonishing scene yesterday as a tranquil seaside resort was plunged into a full-scale terror alert.’ Apart from being gibberish, this is notable for the fact that every noun is weighed down with an adjective. 0/10. Another good example from the Mail on Sunday in March 2013: ‘Despite being covered in huge bites, vets managed to save the dog.’ (In fact the original said ‘vets later managed’, an example of a superfluous ‘later’.)
I/me: These can be confused, especially when used with a second person. For example ‘John and me are going swimming’ and ‘The letter was addressed to my husband and I’ are both wrong. The simple way to sort out which you need is to rephrase it mentally as if it is just the first person. So you would get ‘I am going swimming’ and ‘the letter was addressed to me’. Then put the second person back into the sentence, so ‘John and I are going swimming’, and ‘the letter was addressed to my husband and me’.
indirect speech: It used to be the rule that if you wrote ‘he said that . . .’ the subsequent reported speech had to be in the past tense, such as ‘he said that there was a good chance of recovery’. This has considerably relaxed, and now I think it is better to write ‘he said that there is a good chance of recovery’.
lay: This is the past tense of the verb ‘to lie’ as in ‘I lay on the ground yesterday’ and is also the transitive verb (transitive means it must take an object; it cannot stand alone) ‘to lay’ as in ‘the hen lays eggs’ or ‘I am going to lay the table’. The past tense of ‘to lay’ is ‘laid’. Of course, as everyone knows, ‘lay’ and ‘laid’ are also colloquial sexual expressions and great care must be taken to avoid an inadvertent double meaning. However the chief offence is using ‘lay’ instead of ‘lie’, as in ‘I’m going to lay down’, ‘She is laying on the bed’ or ‘The lion lays in wait for its prey’, or using ‘laid’ instead of ‘lay’, as in ‘He laid on his bed’. To complete the confusion there is the verb ‘to lie’ or tell an untruth. This one is comparatively simple, however.
A brief tour round the tenses:
to lie (as in recline)
present: I lie on the bed, he lies on the bed/I am lying on the bed
past: I lay on the bed, he lay on the ground
participle (with a form of have) I/he/we have/has/had lain on the bed
Note that the word ‘laid’ does not exist in this verb.
to lay (as in to put or place, followed by an object)
present: I lay the table, the hen lays eggs/I am laying the table
past: I laid the table, the hen laid eggs
participle: I/she have/has/had laid the table
Note: this is the only use for the word ‘laid’.
to lie (as in to tell an untruth)
present: I lie, he lies/he is lying
past: I/he lied
participle: I/he have/has/had lied
You will see that there are numerous opportunities for double meanings even if you are being perfectly accurate. If you see such a pitfall looming, at all costs find another form of words. If you are about to use the word ‘lay’ at all, and you are not 100 per cent sure that it is correct, check. There are few errors that betray ignorance as much as this one. Incidentally, British writers use 'lie of the land' while Americans say 'lay of the land'.
none: Usually means ‘not one’ and takes a singular verb, such as ‘None of the subs is likely to make a mistake about this’. However there are occasions when sticking to the rule makes for clumsy reading and ‘none’ would be better treated as meaning ‘not any’. For example ‘Rescuers searched for survivors but none were found’ could be better than ‘none was found’.
only: This is often misplaced in a sentence. It should go as close as possible to the word to which it refers, otherwise the sentence might not say what you mean. This is a classic illustration: ‘I only kissed her because I was drunk’. With ‘only’ next to ‘kissed’ it suggests that I could not do anything more than kiss her because I was too drunk. Move ‘only’ one place and you get ‘I kissed only her because I was drunk’. This suggests I kissed one girl rather than many because I was too drunk to go looking. Move it again: ‘I kissed her only because I was drunk’, which suggests that when sober I would not have touched her with a barge-pole. Don’t go to extremes like one chief sub who changed the heading ‘I only have eyes for you’ to the technically correct ‘I have eyes only for you’.
past tense and past participle: Often confused. For example, sprang is the past tense of the verb to spring, eg ‘she sprang into action’; sprung is the past participle, which goes with ‘has’ or ‘had’, eg ‘the wind has sprung up’. Similarly sink, sank, sunk (also see active/passive, above). The film Honey, I Shrunk the Kids should be Honey, I Shrank the Kids or Honey, I Have Shrunk the Kids.
pluperfect tense: For example, ‘the burglar had appeared in court’. Often unnecessary and the sentence would be better reconstructed in simple past tense. It is necessary only when you are saying that something happened prior to something that you are already describing in the past tense, such as ‘the couple who disappeared last week had met online’. In fact every time you find yourself using the pluperfect you could consider whether you are telling the story in the right order. You should not often have to refer to a previous event if it has appeared in the correct place in the narrative.
shall: This is rapidly disappearing in the face of ‘will’. Traditional practice is that ‘shall’ goes with the first person singular and plural (I shall make it my business, we shall soon be there), ‘will’ goes with the others (he will take office, you will have lunch, they will go to bed at 7pm). These can be reversed for emphasis: ‘I will go the meeting whatever the weather’, or ‘Cinderella, you shall go the ball’.
singular or plural? This can be a really tricky one. Do you say ‘the audience is’ or ‘the audience are’? ‘The jury was told’ or ‘the jury were told’? There are some very involved grammatical rules but in practice the choice usually comes down to a matter of judgment and what sounds best. It often depends on whether you are dealing with a group as a single entity or a collection of individuals. So you could have ‘the audience were silent’ or 'the audience was silent', but you could not have ‘the audience stamped its feet’. Therefore for consistency I would recommend using the plural in every case. Additionally, the smaller the number involved, the more likely it is that you will need the plural, because you start to visualise individuals rather than a mass. For example ‘The family were moved to alternative accommodation’ is better than ‘the family was moved to alternative accommodation’. This is even more marked when you get to ‘a couple’. You simply cannot say ‘A couple is celebrating its silver wedding ’. The golden rule is to stick with whatever you choose. Almost every day you see howlers in this department. Examples: ‘Labour is starting to look unelectable in England following their rout in Norwich North’; (a caption) ‘the troop of monkeys examines their haul’. The best ever: ‘The young family does as they are ordered, packs what they can into plastic bags and joins a growing line . . .’ (August 2010). If you cannot frame a sentence that does not involve the painful clash of singular and plural, then write it in a different way. Note that sporting teams always take the plural, eg ‘England are batting’, ‘Manchester United were beaten’.
singular their/they: This is often used when the writer does not or cannot know if the subject is male or female, for example ‘Each person is entitled to their opinion’. Strict grammarians say this should be ‘Each person is entitled to his or her opinion’. Others say ‘their’ is perfectly acceptable and point out that it has been used by many illustrious writers. I suggest that if ‘his or her’ is grating or needs too much repetition you try to find an alternative, such as recasting the sentence in plural form (‘All people are entitled to their opinions’) or replacing ‘their’ with an indefinite object (‘Each person is entitled to an opinion’), but in the last resort use the singular ‘their’ and don’t worry about it. This all applies equally to the use of ‘they’, as in ‘When someone finds lost property, they should hand it in’.
split infinitive: Many regard splitting an infinitive as the height of ignorance, while others see getting worked up about it as the very definition of pedantry. An infinitive is the basic ‘title’ form of a verb, such as ‘to fly’, and the much-cherished rule is that you do not separate ‘to’ and ‘fly’ with any other word. So instead of ‘to boldly go’ you should say ‘to go boldly’ or ‘boldly to go’. However pro-splitters can, and do, produce lists of examples of breaking the rule from distinguished writers over centuries. I would avoid splitting if possible, if only to prevent retired colonels in Tunbridge Wells from choking on their corn flakes, but slavish adherence to the rule can look pompous and silly. If this is the case, split away (or rewrite the heading or sentence). I will now duck beneath the parapet.
subjunctive: Used to express a wish or a hypothetical situation, for example ‘If I were you’ or ‘He wished he were young again’. The form ‘were’ replaces the usual ‘was’. However this may sound a little formal these days, so use your judgment.
tautology: This is a very frequent error, mainly because it comes in so many guises, often in common use. It means saying the same thing twice in different forms or adding an unnecessary qualifier. For example ‘rise up’, ‘raze to the ground’, ‘personally I think’, ‘personal belongings’, which sets my teeth on edge every time they say it on the train, ‘work colleague’, ‘creating new jobs’, ‘lowering a ladder down’. A frequent one is ‘makeshift/temporary camp’. If it were permanent, it would not be a camp.
which and that: The rules on these two are complex and possibly rather old-fashioned. I think you should use the one that feels better. Some subs were taught to delete ‘that’ whenever they see it, but this is wrong. It is often necessary to make a meaning clear on first reading, for example ‘The police warned that the rioters were out of control.’ If you remove ‘that’ you get ‘The police warned the rioters were out of control’ which makes the reader think at first that the rioters were being warned about something by the police, then have to go back and re-read it. The reader should never be allowed to stumble over the sense of a sentence. ‘That’ is usually needed after words such as ‘suggest’, ‘propose’, ‘warn’ and ‘recommend’. It is not always needed after ‘said’ but in my opinion it can make a better-balanced sentence to include it.
who, whom: The easiest way to determine which to use is to turn the sentence round and replace ‘who’ or ‘whom’ with he/she or him/her, on the basis that ‘who’ equates to ‘he/she’ and ‘whom’ equates to ‘him/her’. So the sentence ‘The man who came into the shop’ becomes ‘He came into the shop’, so you know that ‘who’ is right. If it said ‘The man whom came into the shop’, it would become ‘Him came into the shop’, which would clearly be incorrect. In the same way, the sentence ‘The girl whom I saw last week’ becomes ‘I saw her last week’, which is correct. ‘The girl who I saw last week’ becomes ‘I saw she last week’, which is incorrect. My way of remembering is that ‘him’ and ‘whom’, which go together, both end in ‘m’.
accents (This also appears in Punctuation): A thorny topic. Some feel that French words should have the correct accents. I don’t, providing the meaning is clear. One word where an accent may be necessary is divorcé (the masculine of divorcée, to distinguish from the word divorce). You can get away with divorcee, fiance and fiancee on the grounds that unlike divorcé, they do not have alternative meanings without the accent. Some authorities say that divorcee is fine for both male and female, like payee or addressee. I think you should stick to masculine and feminine versions because they come direct from the French, whereas employee, for example, is the partner word of employer, a different construction altogether. However I don't care for either divorcé or divorcee for a man, and you can easily write ‘Mr Smith, who is divorced . . .’ I have seen a reader’s complaint about the absence of an acute accent on coupe, as in car, which he thought could be confused with coupe, meaning a dessert of fruit and ice cream. Both these words are French in origin, but I defy anyone to come up with a sentence in which a car could possibly take the place of a pudding. The same reader felt that it was wrong not to put accents on ‘menage a trois’. Again, this cannot be confused with anything else. In any case, there never seems to be any suggestion of using accents on Spanish, German or Scandinavian words, so really I cannot see a logical reason for treating French differently.
French: Both noun and adjective are pluralised, eg femmes fatales, bons mots.
Greek: Phenomenon is singular, phenomena is plural. Hyper as a prefix means too much, as in ‘hyperactive’; hypo means too little, as in ‘hypothermia’ or too little heat.
Latin: Not many pupils study Latin these days, but it is useful to know how to handle the many Latin words in the language. Nouns fall into three categories, masculine, feminine and neuter, each with its own singular and plural endings. An example of masculine is one fungus and two fungi. Feminine: one larva (immature insect) and two larvae. Neuter: one bacterium, many bacteria. If you are not sure whether a word with an ‘a’ ending is feminine singular or neuter plural (which you may need to know so that you can team it with a singular or plural verb), you will have to look it up in the dictionary. Having said all that, note that with neuter nouns, in particular, it can look rather precious (if not unintelligible) to be rigid. For example I think you would use not the plural of forum, which is fora, but the technically incorrect forums. The same applies to stadium and ultimatum. I think you would use the correct plural of curriculum vitae, which is curricula vitae, though it would be much easier to say CVs. The plural of memorandum is memoranda, and millennium is millennia (note two ‘n’s). Antennae is the plural of insect feelers, but you would use antennas for aerials. To illustrate that nothing is ever simple, one word which crops up frequently is alumnus, meaning former pupil. This exists in masculine and feminine forms, so you would have one male alumnus and two male alumni, or one female alumna and two female alumnae. A group of mixed gender would be alumni (an example of language biased towards the male). Data is the plural of datum but modern usage (except in some technical and scientific contexts) is to treat it as singular, as in ‘the data shows that . . .’