,1. 表示一个过去发生的动作对现在产生的结果和影响。应该明确现在完成时态属于现在时态的范畴。它所强调的不是过去发生的动作，而是对现在产生的结果和影响。否则用一般过去时态就够了。请看下面的对话。● A－We will send some one to America.我们要派一个人去美国。B－I have been there.我去过美国。= I was there and I know the country very well. Would you send me there? 我去过那儿，很了解那个国家。能派我去吗？C－I have not been there.我还没去过那儿呢。= I didn’t go to America. It is my turn.我没去过。这次该我了。上面对话的前提是大家都抢着去美国。所以B用现在完成时态是想说明自己有经验，派他去好。而C用现在完成时态是说自己没去过，轮也该轮到他了。两句都是强调对现在产生的影响。● A--We will send some one to Africa.B--I have been there.= I was there before. You’d better send another one there.我以前去过那。你最好派别人去。● Mother--What are you doing?Son--I’m watching TV. I’ve done my homework.= I finished my homework. I can watch TV now.儿子说他做作业了。他没有用 I did my homework 而用现在完成时态，主要强调的是对现在产生的结果和影响：他现在可以看电视了。一般过去时态和现在完成时态的比较：● I left my exercise book at home. ● I have left my exercise book at home. =I left my exercise book at home, so I can’t give it to you.第一句一般过去时态只是说把作业本忘在家了。而现在完成时态除此以外更重要的是“我不能交作业了”。所以现在完成时态 = 一个过去时态和一个现在时态。说话人所强调的是现在时态，即对现在产生的结果和影响，但这种结果和影响并没有直接说出来。这正是完成时态比较难理解的地方。● He broke his leg yesterday. ● He has broken his leg. =He broke his leg, so he can’t play football this week. ● I finished the letter. ● I have finished the letter. =I finished the letter. Here it is now. ● He read all these books. ● He has read all these books. =He read all these books, so he knows them and can talk about them. 注：因现在完成时态所关心的是对现在产生的结果和影响，并不在乎过去的动作是过去什么时间发生的，所以不能和表示过去的时间状语连用。因此当问及某一动作曾发生的具体时间时，也不用完成态。例：● When did you get there?╳ When have you got there? 2. 表示过去发生的动作一直持续到现在。通常用于持续性较强的动词，例如：live、study、wait for、be等等，并且常常与since, for连用，since给出的是过去的一个时间点，如since 1958, 而for往往给出的是一个时间段，如for three years。 ● He has studied English for eight years. ● Since 1980 they have known each other.● We have waited for you for long time.● Have you lived here for over ten years?● He has thought about the problem since last week. ● The sheep has lain on the grass for about two hours
