情态动词后为什么不接动词原形

情态动词后一定是接动词原形吗？请看下面的题目：

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

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

此题应选B。容易误选A。简单地认为：在情态动词后永远要用动词原形。

其实这是一个省略句，若把句子补完整应为。如：

He tried his best and did what he could do to help us. 他竭尽全力，做了他所能做的一切来帮助我们。

从上句可以看出：句中的不定式(to help us)是用作目的状语的，而不是与情态动词(could)一起构成谓语。

请比较以下几句。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题应选 A。其中的不定式短语 to catch the early bus 用作目的状语。

第2题应选 C。其中的现在分词短语 hoping to catch the early bus 用作伴随状语。

第3题应选 C。其中的动名词短语 practising spoken English 与 spent 有关(spend... [in] doing sth. )