Here is a direct copy and paste from The Apostrophe Protection Society:
The rules concerning the use of apostrophes in written English are very simple:
1. They are used to denote a missing letter or letters, for example:
- I can’t instead of I cannot
- I don’t instead of I do not
- it’s instead of it is or it has
2. They are used to denote possession, for example:
- the dog’s bone
- the company’s logo
- Jones’s bakery (but Joneses’ bakery if owned by more than one Jones)
This applies to all nouns, so the correct versions are Jesus’s disciples, Keats’s poems and so on.
Please note that “Its”, which is usually used as a possessive adjective (like “our”, “his” etc), does not take an apostrophe:
- the dog ate its bone and we ate our dinner
… however, if there are two or more dogs, companies or Joneses in our example, the apostrophe comes after the ‘s’:
- the dogs’ bones
- the companies’ logos
- Joneses’ bakeries
3. 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 read Bananas for sale
- Menu’s printed to order which should read Menus printed to order
- MOT’s at this garage which should read MOTs at this garage
- 1000’s of bargains here! which should read 1000s of bargains here!
- New CD’s just in! which should read New CDs just in!
- Buy your Xmas tree’s here! which should read 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