The rules concerning the use of apostrophes in written English are very simple:
Apostrophes are used to denote a missing letter or letters
- it's instead of it is
- I can't instead of I cannot
- I don't instead of I do not
- it's instead of it is
Apostrophes are used to denote possession
- the dog's bone
- the council's logo
- Jones's bakery (but Joneses' bakery if owned by more than one Jones)
... but please note that the possessive form of it does not take an apostrophe any more than ours, yours or hers do
- the bone is in its mouth
... however, if there are two or more dogs, councils or Joneses in our example, the apostrophe comes after the 's':
- the dogs' bones
- the councils' logos
- Joneses' bakeries
Apostrophes are NEVER ever used to denote plurals! Common examples of such abuse (all seen in real life!) are:
- Banana's for sale which of course should be written Bananas for sale
- Menu's printed to order which should be written Menus printed to order
- MOT's at this garage which should be written MOTs at this garage
- 1000's of bargains here! which should be wrritten 1000s of bargains here!
- New CD's just in! which should be written New CDs just in!
- Buy your Xmas tree's here! which should be written Buy your Xmas trees here!
Note: Special care must be taken over the use of your and you're as they sound the same but are used quite differently:
- your is possessive as in this is your pen
- you're is short for you are as in you're coming over to my house