I've been practising my serve for weeks.

我练发球有好几周了。

The box will serve for a seat.

这个箱子可以当作一个座位用。

The book will serve for a pillow.

这本书可当个枕头。

The delegates serve for five years.

这些代表任职五年。

I will serve for you!

我将竭诚为您服务！