trousers前面加a吗

不加。

n. 裤子，长裤These trousers are a bit tight.

这条裤子有点紧。

He was dressed in khaki trousers.

他穿着一条卡其色的裤子。

The trousers have a fly fastening.

这条裤子是前开口。

He dropped his trousers.

他脱了裤子。

The seat of his grey trousers split.

他的那条灰色裤子的臀部裂开了。