Check out Beware of False Plurals! by rogersgeorge. Here is an excerpt:
Ordinarily we make a plural in English by adding an “-s” at the end.
Almost immediately, though, things begin to get complicated. Sometimes you have to add “-es” (remember fourth grade?). Then some words don’t change at all to become plural, such as “fish” and “moose.” Some words change a vowel…
The full article is available here.