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。