有when,while,although,as if,unless,whether,where來引導(dǎo)的句子時.且具備：當(dāng)主句和從句的主語相同時,從句主語可以省略.或從句中的謂語是Be的一種形式.或從句中主語是it.
省略句
一.概念
英語中,為了避免重復(fù),常常省略句子中的一個或幾個成分,這樣的句子叫省略句
二.相關(guān)知識點精講
1.簡單句中的省略：
在對話中,交談雙方都知道談?wù)摰膶ο?則可以省略句子的主語,省略主語和謂語的現(xiàn)象在交際用語中出現(xiàn)的很多.
（1）Looks like rain.
（2）Hope to hear from you soon.
（3）Sounds like a good idea.
（4）Beg your pardon.
2.并列句中的省略：
在并列句中,相同的成分如主語,謂語,賓語等都可以省略：
（1）They learn French and we English.
（2）My father planned and built all these houses.
（3）John won the first race and Jimmy the second.
3.復(fù)合句中的省略：
定語從句：
（1）That’s the reason he is late for the conference.
（2）I don’t like the way he talks.
狀語從句：
（1）If heated,water will boil.
（2）Tom was attacked by cramp while swimming across the river.
賓語從句：如果賓語從句中的謂語部分與主句的謂語部分或上文的謂語部分相同,可將從句部分的謂語省略.
（1）We will do what we can（do）to help you.
動詞不定式的省略：在動詞不定式結(jié)構(gòu)中,為了避免重復(fù),常常省略不定式結(jié)構(gòu)中的動詞原形,只保留to.
（1）—Would you like to go with us
—I’m glad to,but I have to finish my homework.
（2）Don’t tell me the name of the sailor if you don’t want to.
在used to,ought to,have to,would like/love to,wish to,be going to等結(jié)構(gòu)中,常常省略to后面的動詞原形.
（1）They do not visit their parents as much as they ought to.
（2）He doesn’t get up early as he used to.
Tell ,warn ,order ,advise ,ask等動詞的賓語后面接動詞不定式做賓語補足語時,可以省略to后面的動詞原形.
（1）He wanted to swim across the river but I warned him not to.
（2）The boy wanted to play football in the street,but his mother told him not to.