Singular and plural forms in English may differ from Portuguese a little bit..
There are words which will change or remain the same in the plural form.
.
Check out a couple of examples below:
Man = Men.
Leaf = Leaves.
Mouse = Mice.
News = News.
Company = Companies
..
Check out the following links to practice plural and singular forms:
.
http://www.englisch-hilfen.de/en/exercises/plural1/index.php
http://www.englisch-hilfen.de/en/exercises/plural2/index.php
http://www.nonstopenglish.com/exercise.asp?exid=230
http://www.english-4u.de/plural_ex1.htm
Best wishes,
André
