英语中if引导的条件状语从句用动词原形还是非谓语动词

英语中if引导的条件状语从句用动词原形

还是非谓语动词

请看下面的句子，空格处应填动词原形还是非谓语动词：

If they don't understand it the first time, _________ over it again until they do.

A. going

B. to go

C. gone

D. go

【分析】此题正确答案选D，其余几项均有可能被误选。空格前if 引导的是一个条件状语从句，而在空格处无论是填A、B或C，它都是一个非谓语动词短语，而不是一个句子，这样一来，整个句子没有主句，从而导致结构不完整。若选D，则go over it again until they do 则是一个句子(祈使句)，在此用作主语。又如：

(1) If he likes to eat it, _________ him some more.

A. give

B. giving

C. given

D. to give

(2) If anyone calls, _________ them I'm not at home.

A. tell

B. telling

C. told

D. to tell

(3) When your mother comes back, _________ her to come to school at once.

A. ask

B. asking

C. asked

D. to ask

(4) While the baby is sleeping, _________ your work as soon as possible.

A. finish

B. finishing

C. finished

D. to finish

答案均选A，空格前分别为if, when, while 引导的状语从句，空格处填动词原形，构成祈使句，用作句子主语。