现在完成时的构成和用法讲解
1. 构成：have / has done
2. 用法：
(1) 过去发生的动作或状态持续到现在(可能还将持续下去)的动作。
     标志词：lately, recently, since then, up to now, so far (至今) , in the last / past few years (在过去的这几年里) ... since +  时间点, for + 时间段 如：
  In the past few years, great changes have taken place in my hometown.
     He has written 8 books so far.
(2) 一件已经发生的事情对现在造成的影响。
     标志词：already, just, yet, never, before... 如：
     He has just turned off the light.
     I have already seen the film.  
(3) This / It is the first / second time + that 从句。that从句的谓语要用现在完成时。如：

     This is the first time (that) I have come here.

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(4) 在条件、时间、让步状语从句中，表示将来某时间以前已完成的动作。如：
     I will not believe you unless I have seen it with my own eyes. (强调看完)
     I will go with you as soon as I have finished my work. (强调做完)
3. 现在完成时和一般过去时的区别：
 现在完成时强调的是影响；一般过去时强调的是动作。如：
He has cleaned the classroom.  他打过扫教室了。（教室已经干净了,不用再扫了）
He cleaned the classroom. 他打扫教室了。（现在干不干净，不知道）
I have lost my new book. 我把新书丢了。(现在还未找到)    
I lost my new book yesterday. 我昨天把新书丢了。(昨天丢的，现在找到与否没说明)

现在完成时可以和带有since或for等表示 “一段时间” 的状语连用, 表示动作或状态从某一时刻开始, 一直持续到现在。但是, 像come, arrive, buy等终止性动词不能与表示 “一段时间” 的状语连用。如果要用，必须延续性动词来表述。如：
His grandfather died two years ago.
→ His grandfather has been dead for two years. 他爷爷去世两年了。
He left five minutes ago.
→ He has been away for five minutes. 他离开五分钟了。

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