what they could do 还是what they could to do
The soldiers tried their best and did what they could _________ against the flood.
A. fight B. to fight C. fighting D. fought
【分析】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 fight，即断定选A答案。其实选错了，正确答案应是B。这是一个省略句，即在 could 后省略了动词 do，若把句子补完整应为The soldiers tried their best and did what they could do to fight against the flood (士兵们竭尽全力抗洪)。即句中的不定式短语 (to fight against the flood) 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) He did what he could to help us. 他尽力帮助我们。
(2) He saved what he could to buy a car. 他尽量节约每一分钱以便买车。
(3) He ran as fast as he could to catch the early bus. 他拼命地跑以便赶上早班车。
(4) He ran as fast as he could _________ to catch the early bus.
A. to hope B. hope C. hoping D. hoped
若根据以上“经验”认为此题要选A答案，那就又错了。此题应选 C，其中的现在分词短语 hoping to catch the early bus 用作伴随状语。
(5) He spent every minute he could _________ spoken English.
A. practise B. to practise C. practising D. practised
【分析】此题应选 C，它与以上各题的情形均有所不同，虽然与上面的第(4)题的答案一样，均为动词的 –ing 形式，但也不完全一样：第(4)的 hoping 为现在分词，在句中用作伴随状语；而此题的 practising 是动名词，之所以用它与前面动词 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) Mr Smith made up his mind to devote all he could _________ his oral English before going abroad.
A. improve B. to improve C. improving D. to improving
【分析】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。