Possessive Case

We use the possessive case ('s or ') to express that something belongs to someone.

Let's look at the rules:

1 - With plural nouns, we add: '
      e.g. The scientists' theory.

2 - With words without the final s, we add: 's
      e.g. The teeth's colour.  Tim's work.

3- With 2 or more names, we had 's to the last name we write.
     e.g. Tim, Brenda and Alan's grandmother.

4 - With people's names which end in "s" we add: 's
      e.g. Charles's bike.   Inês's car. 


Visit these websites:
http://www.ego4u.com/en/cram-up/grammar/nouns/exercises?04
http://www.englisch-hilfen.de/en/exercises/nouns_articles/apostrophe_s.htm