1. Apostrophes show ownership:
- Owned by one person - Dad's car - the apostrophe comes before the 's'
- Owned by more than one person - the boys' boat - the apostrophe comes after the 's'
2. Apostrophes indicate an abbreviation:
- Have not becomes haven't
- Is not becomes isn't
- Should not becomes shouldn't
3. Its and it's kind of break the rule:
- It's is short for 'it is' (it's a hot day)
- Its shows ownership (the dog chased its tail)
4. Apostrophes are not used whenever you see an 's'. An 's' simply makes something plural. It doesn't indicate ownership or an abbreviation. You'll see apostrophes used like this (below) all the time. They're wrong.