The humble apostrophe causes all sorts of problems, albeit through no fault of its own, but the only reasons to use an apostrophe are if a letter is missing, or to show something belongs to someone. You can't go far wrong if you remember:
PLURALS and PRONOUNS DON'T but MINE and MISSING DO.
A plural doesn't need an apostrophe. In most cases, you just add an 's' to the end of the word.
The two dogs licked their paws.
Two dogs, eight paws – no apostrophes.
If the word is made up of letters, the rule is still the same.
The two dogs licked their paws as they sat waiting for the five GPs and the
seven MPs to finish their meeting.
Two dogs, eight paws, five GPs, seven MPs – no apostrophes.
A pronoun is a word that is used instead of a proper name. There is no need to add an apostrophe in his, hers, ours, yours, its, theirs or whose.
Two GPs asked the first five MPs if the dogs were theirs, while the last two MPs
pointed to a woman in the corner and said they were hers.
Two dogs, eight paws, five GPs, seven MPs, two of the GPs asking questions, two of the MPs answering and a lady with two dogs – no apostrophes.
If something belongs to someone, you need to add apostrophe then 's'.
The lady's dogs looked out of the windows and saw the first GP's cat sitting
in the tree cleaning its whiskers.
The dogs, the windows and the whiskers are plural and the 'its' is a pronoun – no apostrophes. The lady owns the dogs and the GP owns the cat – so they both need apostrophes.
If it is a plural noun, the rule is the same: the apostrophe goes after the word and before the 's'.
A man and a woman walked past carrying some children's trousers in two bags.
The doctors laughed as the two people's hats blew away in the wind.
Plural trousers, bags, doctors and hats – no apostrophes. The hats belong to the people, which is already a plural noun, so apostrophe then 's'.
If the noun is already plural so ends in an 's', the rule is the same – the apostrophe goes after the word but it only sometimes needs the extra 's'. Say it to yourself and it will be obvious.
Then four more cats came along and sat on two wooden seats.
Three cats' tails swished back and forth but one cat's tail was wrapped around itself. The MPs put on their coats and saw that the first doctor's coat was brown but
the other doctors' coats were black. One of the doctors said the brown coat
could be James's or Peter's but Dr Hastings' coat was definitely black.
So now we have plural cats, seats, tails, MPs, doctors, coats and a prounoun – no apostrophes. One cat owning its tail, one doctor owning his or her coat, Peter and James each owning their coats – apostrophe 's'. Several cats owning their tails and doctors owning their coats – apostrophes after the 's'. Mr Hastings owns his coat, too, but you wouldn't say 'Mr Hastings's coat' so you drop the final 's'.
When you run two words together, you replace the missing letter with an apostrophe. To tell whether or not it needs an apostrophe, say the word in full, so 'haven't' is short for 'have not' with the 'o' missing.
It's a very silly story but if you haven't got the idea yet, it'll soon be etched on your brain when you think about how you'll tell others the right way to use apostrophes.
Just remember PLURALS and PRONOUNS DON'T but MINE and MISSING DO.