do, doing还是to do

请看这样一道题：

Everyone in our class was working hard and doing what we could _______ a good college.

A. enter                            B. to enter

C. entering                        D. entered

【陷阱】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 enter。

【分析】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Everyone in our class was working hard and doing what we could do to enter a good college。即句中的不定式短语(to enter a good college)是用作目的状语的，而不是与情态动词(could)一起构成谓语。请再看类似例句：

(1) They did what they could to comfort her. 他们尽量安慰她(from www.nmet168.com)。

(2) They did everything they could to save her life. 他们尽一切力量拯救她的性命。

(3) He studies as hard as he could to catch up with his classmates. 他拼命学习以便赶上他的同学。

值得指出的是，这也并不是说今后只要遇到类似结构的题就一定选带to不定式。请看以下试题：

(4) He ran as fast as he could _____ to catch the early bus.

A. to hope                        B. hope

C. hoping                         D. hoped

此题的答案是 C不是A，其中的现在分词短语 hoping to catch the early bus 用作伴随状语。

(5) He spent every minute he could _____ spoken English.

A. practise                        B. to practise

C. practising                     D. practised

此题答案选 C，这与前面动词 spent 的搭配有关，即 spend … (in) doing sth。若将此句补充完整，即为 He spent every minute he could spend in practising spoken English.

(6) Before going abroad he devoted all he could _______ his oral English.

A. improve                        B. to improve

C. improving                     D. to improving

此题答案选D，注意两点：一是 devote … to … 是固定搭配，意为“把……贡献给……”；二是其中的 to 是介词，不是不定式符号，后接动词时要用动名词(from www.nmet168.com)。