You have front pockets and you have back pockets in a pair of pants.

一条裤子有前口袋和后口袋。

You'll often have pockets in the back of the pants – to put a wallet, for example – at least, most men's pants have a pocket in the back.

裤子的后部通常会有口袋——例如，用来放钱包——至少，大多数男士裤子的后部都有一个口袋。