Apostrophe '

• To show a possession

  Examples:

  Mark’s book
  She and Mark’s car cost a lot.
  Children’s book
  Students’ book
  Fifteen-years-old’ behavior
  
  To form plurals of numbers and letters
  Examples:
   
  He saw ten W’s and five 7’s in this letter. 
  
   To indicate omitted letters in contractions 
  Examples: 
  I'm, you’re, he’ll, they’ve, it’s, he didn’t . . ..