In languages such as French, it is normal for an adjective to be placed after the noun it modifies. This is far less common in English. When an adjective appears after the noun it modifies, it is said to be postpositive. This is as opposed to prepositive (i.e. "normal") adjectives, which appear before the noun being modified.
Wikipedia summarises this concept nicely:
Of course, like many things in language, we use postpositive adjectives passively, without any additional thought or analysis or even realising that the word we've just said is an adjective. Consider, for example, the sentence The people present in the room. At first glance, it isn't immediately obvious that "present" is an adjective. But it is. (In this example, at least.) And "people" is the noun. This is the sort of thing we'd say, probably oblivious to the fact that it is constructed using a postpositive adjective.In some languages the postpositive placement of adjectives is the normal syntax, but in English it is less usual, largely confined to archaic and poetic uses (as in They heard creatures unseen), phrases loaned from Italic languages, (such as heir apparent, aqua regia), and certain particular grammatical constructions (as in those anxious to leave).
So what about the plurals? Well, again, that's to do with postpositive adjectives. To use Wikipedia's example: "court-martial". It becomes "courts-martial" in the plural. That's because the noun is the first part of the compound expression. It wouldn't really make sense to pluralise an adjective. Well, at least not under the rules of English. Unlike other languages, adjectives in English almost never agree with the nouns being modified. I suppose when you think about it, it's interesting that this is the case, given all the intricacies of the English language (and the fact that a lot of our language comes from French).