有when,while,although,as if,unless,whether,where来引导的句子时.且具备：当主句和从句的主语相同时,从句主语可以省略.或从句中的谓语是Be的一种形式.或从句中主语是it.
省略句
一.概念
英语中,为了避免重复,常常省略句子中的一个或几个成分,这样的句子叫省略句
二.相关知识点精讲
1.简单句中的省略：
在对话中,交谈双方都知道谈论的对象,则可以省略句子的主语,省略主语和谓语的现象在交际用语中出现的很多.
（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,相同的成分如主语,谓语,宾语等都可以省略：
（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.复合句中的省略：
定语从句：
（1）That’s the reason he is late for the conference.
（2）I don’t like the way he talks.
状语从句：
（1）If heated,water will boil.
（2）Tom was attacked by cramp while swimming across the river.
宾语从句：如果宾语从句中的谓语部分与主句的谓语部分或上文的谓语部分相同,可将从句部分的谓语省略.
（1）We will do what we can（do）to help you.
动词不定式的省略：在动词不定式结构中,为了避免重复,常常省略不定式结构中的动词原形,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等结构中,常常省略to后面的动词原形.
（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等动词的宾语后面接动词不定式做宾语补足语时,可以省略to后面的动词原形.
（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.
