Why do we use the singular form when we're clearly referring to something that's plural?

English is one of the hardest languages to learn because of those examples =) This usually happens because we speak differently then we write. We tend to speak in a way that is most convenient, and right now, that means say food instead of foods, and youth instead of youths. Besides that, while languages like french tend to make rules and stick to them, almost every rule that we learn when learning english has a dozen exceptions. For example name 10 verbs off the top of your head, and then look at the past tense fr those verbs- at leat 4 will be irregular. =D