文档介绍：一、时态讲解:现在完成时
(1)构成:现在完成时由助动词 have(has)+动词的过去分词构成。has用于第三人称单数,have用于其他各种人称. 
现在完成时的否定式直接在助动词后面加上not、疑问式是把助动词提到主语之前。以study 为例,其否定式、疑问式和简单回答形式如下:
否定式
疑问式
I have not (haven’t) studied….
Have I studied…?
You have not (haven’t) studied….
Have you studied…?
He has not (hasn’t) studied….
Has he studied…?
否定疑问式
简单回答(肯定/否定)
Have I not (Haven’t I) studied…?
Yes, you have. No, you haven’t.
Have you not (Haven’t you) studied…?
Yes, I have. No, I haven’t.
Has he not (Hasn’t he) studied…?
Yes, he has. No, he hasn’t.
其用法主要有三种 
I.“已完成”用法 :表示一个过去发生并结束的动作对现在造成的影响或结果。
这一类情况可以细致分为下述两种情况。
 1)表示开始于过去的动作刚刚结束。常和just,now,already,yet,not…yet等不确定的时间状语连用。
 Li Ming has just turned off the light. 李明刚刚把灯关上。(说明现在灯关上了) 
I've finished my homework now. 现在我已经做完作业了。(说明可以交作业或做别的了) 
2)表示过去动作的结果,现在仍残留着。一般不用时间状语。 
I have lost my pen. 我把笔丢了。 (说明过去某时丢的,现在我还没找到这支笔)
She has e a teacher. 她已经当了老师。(说明她现在仍是老师) 
 II.“未完成”用法。表示动作或状态从过去某时开始,持续到现在,可能继续下去,也可能刚刚结束。常和表示一段时间的状语连用。如today,this week(month),lately,recently,these days,in the past few days,during the last two weeks,since,since yesterday,since 2 days ago,since 1991,for a long time,for a month,so far,up to now,till(until) now等。 
He has lived here for 30 years. 他住在这儿三十年了。(现在还住在这儿) 
They've known each other since childhood. 他们从小彼此相识。(现在还在往来)
 How long have you studied English? 你学英语多久了?(现在仍在学)
 :表示从过去开始到目前为止这段时间中反复发生的动作或多次出现的状态。
常与频度副词如often,always,every week,twice等连用。
I have been to the Summer Palace twice. 我曾经去过颐和园两次。 
He has always said .
(2)用法:
1)现在完成时通常表示在说话之前已经完成的动作或存在的状态。说话人强调的是该动作或状态对现在的结果或影响。
My daughter has just gone out. 我女儿刚出去。
I’m sure we’ve met before. 我肯定我们以前见过面。
She has arrived. 她到了。
2)表示持续到现在的动作或状态,往往和包括现在在内的表示一段时间的状语连用,如recently, already, just, lately, for…, since…,yet等。如:
I haven’t heard from her these days. 这些日子我没有收到她的信。
We haven’t seen you recently. 最近我们没有见到你。
They have been away for two years. 他们离开已经两年了。
She has been with us since Monday.
★注意:
since和for的区别
since后接时间点,如1993,la