The apostrophe looks like a raised comma and causes almost as much confusion. It originates in the Greek rhetorical device ‘apostrephein’, in which the speaker addressed not the audience but some absent thing or person. It now indicates something missing, usually letters, and has two basic uses:
To indicate possession:
normal singular words – a dog’s dinner (dinner of the dog), his master’s voice (voice of the master), Jane’s book (book belonging to Jane), the lawyer’s wig (wig of the lawyer).
in singular words ending in ‘s’ – Corporal Jones’s bayonet, St James’s Street, the princess’s crown, the dress’s colour,
in plural possessives not ending in ‘s’ the apostrophe goes before the ‘s’– the people’s flag, policemen’s ball
in plural possessives that end in ‘s’ the apostrophe goes after the ‘s’ – birds’ nests, the dogs’ kennels, the girls’ uniforms
To mark contractions (missing words or letters)
she’ll (she will), wouldn’t (would not), you’re (you are), they’re (they are), he would’ve (would have – not ‘would of’), it’s (it is)
The exceptions? Occasionally to mark plurals that would be confusing if left unmarked: e.g. Dot your I’s and cross your T’s
When NOT to use them?
for acronyms, such as EAs, PAs and RFTs (Requests for Tenders) etc.
Finally my pet hate, they are definitely NOT used for words in the plural, e.g. to say ‘word’s in the plural’!