1. 现在完成时：
a) 表示动作刚刚结束（常和just, already, yet等词连用）；或表示动作的结果（一般不用时间状语）。e.g. Jefferson has just turned off the light.、
b) 表示过去某时开始的动作一直延续到现在，并且可能会继续延续下去（常用since引导的短语或从句，或由for 引导的短语连用）。 e.g. The astronaut has been engaged in the research for 30 years. // They’ve known each other since childhood.
c) 非延续性动词的完成时和it is +时间+since…..（过去时）
英语中有些动词不能延续， 因此不能和表示延续的时间状语连用。 e.g. It is (has been) an hour since the delegation arrived here. //It is 3 years since the diver left Shanghai.
d) have (has) been to 和have (has) gone to 的区别
have (has) been to:去过某地（表示某人的一种经历），可以和once, twice, often, never, ever连用。
have (has) gone to: e.g. The novelist has gone to the Yellowstone National Park.// The novelist has been to the Yellowstone National Park twice.
非延续性动词的用法说明：
come e.g. He came here last week. //He has come here.
be here e.g. He has been here for ten months since last year.
arrive: be here
die e.g.The professor died last year. //The professor has died.
be dead e.g. The professor has been dead for one year.
join e.g.Robert joined the army in 2005.//Robert has joined the army.
be in the army e.g. Robert has been in the army for two years.// Robert has been in the army since 2005.//It’s (has been) two years since he joined the army.
buy e.g. Bob bought the dictionary in May.//Bob has bought the dictionary.
get, have e.g. Bob has had the dictionary for four months.
go e.g. The delegation has gone to Japan.//The delegation has been to Japan.
be there, be here e.g. The delegation has been in Japan for three years.
2. 过去完成时 过去的过去
a) 表示在过去某一时间或动作之前已经完成了的动作（即过去的过去）。这个过去的某一时间可用 by , before等介词或连词引导的短语或一个从句来表示。
e.g. by the end of last year, the chemist had worked here for five years.
b) 表示从过去某一时间开始，一直持续到另一个过去的时间的动作。
e.g. The meeting had been on for two hours when we arrived there.
c) 在含有as soon as 等连词引导的状语从句的复合句中，主句和从句都可以用一般过去时表示，而不用过去完成时。
e.g. I called him as soon as I got ticket of Beijing 2008 Olympic Games.
d) 过去完成时常用在no sooner …than…, hardly/scarcely/barely/rarely …when…，“一…就…”句型之中，句子倒装。
e.g. Scarcely had he opened the door when a gust of wind blew the c and le out.
