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。