There are a number of rules for forming plural nouns in English. The basic rule is 'add +s' to the noun (i.e. tree - trees). Here are two quizzes focusing on plural forms. The first, plural forms quiz focuses on standard plurals, as well as a few irregular forms. The second irregular plural forms quiz is more appropriate for advanced English learners.
You may also want to study the plural forms spelling rules a bit as well.