1.表示裤子条数时用pair。
2.trousers修饰其他名词作定语时用单数形式。

He wore a pair of black trousers.他穿着一条黑色裤子。
The two pairs of trousers are mine.这两条裤子是我的。
He took a piece of chalk from his trouser pocket.他从裤袋里掏出一支粉笔。

trousers, bloomers, jeans, pants
参见pants条。