情态动词后为什么接带to不定式

Now everyone here was working hard and doing what they could _______ more money.

A. make            B. to make             C. making           D. made

【答案与分析】

此题容易误选A。有的同学一看到空格前的情态动词 could 就断定此处应填动词原形 make。其实正确答案应是B，此句为省略句，即在 could 后省略了动词 do，若把句子补完整应为Now everyone here was working hard and doing what they could do to make more money，即句中的不定式短语(to make more money)是用作目的状语的，而不是与情态动词(could)一起构成谓语。

【类似考题】

(1) My parents did what they could _____ my aunt and uncle.

A. help               B. to help

C. helping          D. helped

(2) He read what he could _____ his knowledge.

A. widen           B. to widen

C. widening       D. widened

以上两题均选B，即选不定式，且此不定式为目的状语。但是，同学们若据此形成思维定势，也会麻烦。为此，请再看两题：

(3) He ran as fast as he could ____to catch the bus.

A. hope             B. to hope

C. hoping         D. hoped

此题的最佳答案不是B，而是C，其中的现在分词短语 hoping to catch the bus 用作伴随状语，即一边跑一边希望能赶上早班车。但是，如果选B，将 to hope to catch the bus 视为目的状语行不行呢？不行。因为“他拼命地跑”目的是“为了赶上公共汽车”，而不是“为了希望赶上公共汽车”，换句话说，将“希望”作为“目的”不妥。因为同样的原因，下面一题也应选 hoping：

He studied as hard as he could _____ to enter a good college.

A. hope             B. to hope

C. hoping         D. hoped

(4) He spent every minute he could ______ foreign languages.

A. study              B. to study

C. studying         D. studied

此题也应选C，但是，它与上题选现在分词 hoping 表伴随有所不同，此题的 studying 其实与其前的动词 spend 有关，即套用的是“spend + 时间或金钱 + (in) doing sth”这一句型。