a waste of time为什么用a

a waste of time是固定搭配，a是修饰waste的；表示浪费时间的意思；

例句

In my view it was a waste of time.

依我看，这是浪费时间。

Its a waste of time and energy.

那是浪费时间和精力。

\'It\'s a waste of time,\' he concluded

“这是浪费时间。”他总结道。

Personally I think it\'s a waste of time

在我看来，这是浪费时间。

It\'s a waste of time to concern yourself over other people\'s troubles.

为别人的麻烦事操心，纯粹是浪费时间。