And if you want to get even more confused... there are cases when the Japanese are transliterate English words, they sometimes pluralize it -- even if it is intended to be singular.
Fruit --> Fruits (e.g. "fruit juice" becomes "fruits juice")
suit --> suits (e.g. "suitcase" becomes "suitscase")
peanut --> peanuts (e.g. "peanut butter" becomes "peanuts butter")
nutmeg --> nutsmeg
taco --> tacos
Part of the time it's because of the phonetics, and other times it's to avoid abiguities caused from transliteration. The last one -- "tacos" is primarily there to avoid confusion -- "tako" is octopus. The pluralization is used to indicate the Mexican (actually Texan IIRC) food.

Anyway... an article regarding English pluralization:
http://www.absoluteastronomy.com/encyclopedia/E/En/English_plural.htm