COMMON MISTAKES

You make a lot of mistakes when forming irregular plurals. So, here you have a list of irregular plurals:

SINGULAR -- PLURAL
CHILD --- CHILDREN
MAN --- MEN
WOMAN --- WOMEN
PERSON --- PEOPLE
TOOTH --- TEETH
FOOT --- FEET
FISH --- FISH
GOOSE --- GEESE
MOUSE --- MICE
SHEEP --- SHEEP

Here you have some exercises:
http://www.ego4u.com/en/cram-up/grammar/nouns/exercises?03
http://www.tolearnenglish.com/exercises/exercise-english-2/exercise

http://www.englisch-hilfen.de/en/exercises/nouns_articles/plural2.htm

http://www.englisch-hilfen.de/en/exercises/nouns_articles/plural3.htm

If you need more information on the topic, click here.