现在完成时 强调对现在的影响,一般和 过去的段时间连用.
例如:Mike has been away for a long time.
过去完成时 往往发生在过去某个动作前,所谓"过去的过去?,经常和一般过去时态的 状语从句（尤其是时间状语从句）连用
例：He had worked in the firm before he came here.
现在完成时枣表示到目前为止已经完成,并对现在留下某种后果和影响的动作.
结构：助动词have/has + 动词的过去分词 用法:1.表示过去发生的某一动作对现在造成的影响或结果,常与一些时间状语,如:already,yet,ever,never,just,before,once,twice等连用,也可以和包括现在在内的时间状语,如these days,today,this year,so far等连用.
2.表示过去发生的动作或存在的状态,一直持续到现在,也许还将继续下去,可以和表示从过去某一时刻延续到现在的一段时间的状语连用,即for +时间段,since+时间点/从句,in the last ten years等,谓语只可用延续性动词.
4.现在完成时和一般过去时的区别
现在完成时所表明的是过去发生的动作对现在的影响,强调的是现在的情况,不可以和表示过去的时间状语yesterday,in 1991,three days ago last time,last night等连用.
5.现在完成时中的 for 与 since
for + 时间段 与延续性动词的现在完成时连用
since + 时间点/从句
I have kept the library book for a week.= I have kept the library book since a week ago.
6.It is /has been +一段时间 + since 从句.自从某事发生已有一段时间了.
eg.It is /has been two years since my brother joined the army.
过去完成时枣表示过去某一时间或动作之前已完成的动作
结构：助动词had + 动词的过去分词
用法：1.表示在过去某一时间或动作之前已经发生或完成了的动作,动作发生的时间为“过去的过去“.表示过去某一时间可用by,before 等构成的短语,也可用when,before等引导的从句或通过上下文表示.
eg.By the end of last term,we had learned a thousand English words.
He had finished his homework before his father came back last night.
2.表示过去某个时间之前发生的动作或状态,一直延续到这一过去时刻,或还将继续下去.
eg.When I got to know him,he had worked there for twenty years.
(注意) 否定式 had not == hadn't
时态的呼应
1.主句中谓语动词如果是现在时或将来时,从句中的谓语动词可以用任何所需要的时态,
he wants to speak to the headmaster.He said that he won't be free tomorrow.
he came last night.
2.主句中的谓语动词如果是过去时态,从句中的谓语动词一般情况下要用过去时态的某种形式.
He said that he was working hard on his Chinese.
he had a very good journey home.
he hadn't bought the present yet.
he would stop in Moscow on the way.
3.如果从句所述内容是客观真理,则谓语动词总是用一般现在时,而不受主句中谓语动词时态的影响.
eg.Miss Gao told us that light travels much faster than sound.
4.时间或条件状语从句中,若主句是一般将来时或祈使句,从句要用一般现在时代替将来时.
eg.Stop talking when the headteacher ______(walk)into the classroom.
Jim won't watch TV until he ___ (finish) his homework every time.
