一、在对话、交谈中
1. 回答问题时，往往可省略从句或主句中的某些成分、甚至整个从句或主句。如：
(1) —Where were you when the fire broke out last night? 昨天晚上发生火灾时，你 在哪?
—(I was ) In Li Lei’s. 在李蕾家。
(2) —What will you do when you finish your homework? 你做完作业后干什么?
—I’ll go to play football with some friends. 我去跟朋友踢足球。
(3) —What are you going to be when you grow up? 你长大后打算干什么?
—I’m going to be a businessman. 我打算去做生意。
2. 提出建议、劝告、警告或许诺等时，也可省略句子中的某些成分。如：
(1) Work harder, and you’ll succeed. 工作再努力些，你就会成功。
(2) Take care when you walk on the ice. 在冰上行走时要小心。
(3) Hurry up, or you’ll be late. 快一点，不然就会迟到了。
二、某些句型中
1. 表示时间、条件、让步、方式等状语从句中，如果从句的主语跟主句的主语一致或从句的主 语是it，而谓语动词是系动词be时，就可将从句的主语及系动词 be 省略。如：
(1) Solid can be turned into liquid and liquid into gas if heated enough. =solid can be turned into liquid and liquid can be turned into gas if it is heated enough. 如果加到足够 的热，固体会变成液体，而液体又会变成气体。
(2) Have a little more if delicious. =You have a little more if it is delicious 如果 好吃就多吃一点。
(3) Make changes where necessary. =You make changes where it is necessary. 在必要的 地方作些修改。
(4) I won’t attend his party, though invited a few days ago. = I won’t attend his party, though I was invited a few days ago. 尽管几天前请了我，但我还是不会参加他 的聚会。
2. 表示时间、条件、让步、方式等状语从句中，如果从句的主语跟主句的主语一致，就可将从 句的主语省略，同时，将从句中的谓语动词变为分词形式。如：
(1) He was very excited when told that he had passed the exam. =He was very excited when he was told that he had passed the exam. 当告诉他考试通过了时，他非常兴奋。
(2) If running a little faster, he will take the first place. =If he runs a little faster, he will take the first place. 如果再跑快一点，他就会获得第一名。
(3) They began to talk warmly as if being good friends. =They began to talk warmly as if they were good friends. 他们就好像是好朋友一样开始热烈地谈论起来。
