情態動詞后為什么不接動詞原形

情態動詞后一定是接動詞原形嗎？請看下面的題目：

He tried his best and did what he could __________ us.

A. help                         B. to help                        C. helping                         D. helped

此題應選B。容易誤選A。簡單地認為：在情態動詞后永遠要用動詞原形。

其實這是一個省略句，若把句子補完整應為。如：

He tried his best and did what he could do to help us. 他竭盡全力，做了他所能做的一切來幫助我們。

從上句可以看出：句中的不定式(to help us)是用作目的狀語的，而不是與情態動詞(could)一起構成謂語。

請比較以下幾句。如：

1. He ran as fast as he could ________ the early bus.

A. to catch                 B. catch                    C. catching                 D. caught

2. He ran as fast as he could _________ to catch the early bus.

A. to hope                  B. hope                     C. hoping                    D. hoped

3. He spent every minute he could _________ spoken English.

A. practise                  B. to practise             C. practising               D. practised

第1題應選 A。其中的不定式短語 to catch the early bus 用作目的狀語。

第2題應選 C。其中的現在分詞短語 hoping to catch the early bus 用作伴隨狀語。

第3題應選 C。其中的動名詞短語 practising spoken English 與 spent 有關(spend... [in] doing sth. )