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動詞原形還是非謂語動詞

請看下面的句子，空格處應填動詞原形還是非謂語動詞：

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】此題正確答案選D，其余幾項均有可能被誤選�？崭袂� if 引導的是一個條件狀語從句，而在空格處無論是填A、B或C，它都是一個非謂語動詞短語，而不是一個句子，這樣一來，整個句子沒有主句，從而導致結構不完整。若選D，則 go over it again until they do 則是一個句子(祈使句)，在此用作主語。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

答案均選A，空格前分別為 if, when, while 引導的狀語從句，空格處填動詞原形，構成祈使句，用作句子主語。