(三)现在完成时

　　1. 在时间、条件状语从句中，用现在完成时代替将来完成时。

　　I’ll give this dictionary to you as soon as I have finished reading it.

　　If you have done the experiment, you will realize how difficult the theory is.

　　2. 现在完成时可用于下列句型：

　　This /That /It is the first … that …

　　This /That /It is the best … that …

　　This /That /It is the only … that …

　　This is the first time that I have played table tennis.

　　It is the only party that I’ve ever really enjoyed in my life.

　　That is the most interesting film that I’ve ever seen.

　　3. 无论since作介词还是连词，句子应用完成时。

　　Great changes have taken place since the reform and opening to the world.

　　I haven’t seen her since I last saw her.

　　4. 当since, before作副词用置于句末时，应用完成时。

　　I haven’t studied English before.

　　We haven’t met each other since.

　　5. 在it is … since结构中，主句的谓语通常用一般现在时，也可用现在完成时，但从句通常为终止性动词的过去时。

　　It’s a long time since I saw you last time.

　　It is four years since Tom left school.

　　6. 终止性动词appear, begin, set up, buy, die, join, start, come, fall, finish, leave, lend, sell, lose, stop, etc.的完成时不能与for连用。

　　不能说：He has got up for 2 hours.

　　只能说：He got up 2 hours ago.

　　He has been up for 2 hours.

　　It is 2 hours since he got up.

　　7. by + 将来时间状语用将来完成时，by + 现在时间状语用现在完成时。

　　By this time next year we’ll have graduated.

　　By now I have seen the film three times.

　　(四)过去完成时

　　1. by + 过去时间状语用过去完成时。

　　By last year I had learned 2000 English words.

　　2. 在no sooner … than, hardly … when, scarcely … when, barely … when等句型中，主句中的谓语用过去完成时，从句中的谓语用过去时。

　　No sooner had we got all the crops in than it began to rain.

　　She had hardly finished his speech when the audience started cheering.