现在完成时 和过去完成
现在完成时 和过去完成
　定义现在完成时用来表示之前已发生或完成的动作或状态，其结果的确和现在有联系。动作或状态发生在过去　但它的影响现在还存在；也可表示持续到现在的动作或状态。其构成： have (has) +过去分词。 
用法：
1） 现在完成时所表示的动作在说话之前已完成，但对现在有影响，句中没有具体的时间状语。此时多于never， ever， just，before，already，yet等连用。
2） 现在完成时所表示的动作开始于过去，持续到现在，也许还会继续持续下去。常与for，since等一段时间状语连用。
注意：表示暂短动作的词（如come，go，borrow，lend，die，bur，marry等）的完成时不能与for，since等表示一段时间的状语连用。
例如：How long may I keep the book？这本书我可以借多久？
Two weeks 
Borrow和lend 都是暂短性动词，当表示借多久时，用动词keep
常见的短暂性动词转换成延续性动词
leave --- be away,  borrow --- keep,  buy --- have,  begin/start --- be on,  die --- be dead,      finish --- be over,  join --- be in+组织机构, 
be a member of+组织机构,    open sth --- keep sth open,  
fall ill --- be ill,    get up---be up,    catch a cold --- have a cold,  
come here --- be here,    go there --- be there,    become --- be,   
come back --- be back,      fall asleep --- be asleep,  
get to/ arrive/reach --- be (in),      leave --- be away from,  
get to know --- know, go (get) out →be out，  
put on→ wear；catch a cold →have a cold等。   
例:The old man died 4 years ago.  ----The old man has been dead for 4 years.    ---- It is 4 years since the old man died.  ----Four years has passed since the old man died.      
 He joined the Party 2 years ago.  -----He has been in the Party for 2 years.      I bought the book 5 days ago.  ---- I have had the book for 5 days.
  比较过去时与现在完成时 
    1）过去时表示过去某时发生的动作或单纯叙述过去的事情，强调动作；现在完成时为过
去发生的，强调过去的事情对现在的影响，强调的是影响。 
    2）过去时常与具体的时间状语连用，而现在完成时通常与模糊的时间状语连用，或无时间状语。
  一般过去时的时间状语:
  yesterday, last week，…ago, in1980, in October, just now, 具体的时间状语
  现在完成时的时间状语
　for, since, so far, ever, never, just, yet,　till / until, up to now, in past years, always,
　3）现在完成时可表示持续到现在的动作或状态，动词一般是延续性的，如live, teach, learn, work, study, know. 
    过去时常用的非持续性动词有come, go, leave, start, die, finish, become, get married等。
举例：
　I saw this film yesterday.（强调看的动作发生过了。）
  I have seen this film. （强调对现在的影响，电影的内容已经知道了。）　 
　句子中如有过去时的时间副词（如 yesterday, last, week, in 1960）时，不能使用现在完成时，要用过去时。
    （错）Tom has written a letter to his parents last night.
    （对）Tom wrote a letter to his parents last night. 
句子中如有过去时的时间副词（如 yesterday, last, week, in 1960）时，不能使用现在完成时，要用过去时。
    （错）Tom has written a letter to his parents last night.
    （对）Tom wrote a letter to his parents last night.
用一般过去时代替完成时
  1)　两个动作如按顺序发生，又不强调先后，或用then，and，but 等连词时，多用一般过去时。
    When she saw the mouse，she screamed.
    My aunt gave me a hat and I lost it.
  2 ) 两个动作相继发生，可用一般过去时；如第一个动作需要若干时间完成，用过去完成时。
    When I heard the news, I was very excited.
  3)　叙述历史事实，可不用过去完成时，而只用一般过去时。
    Our teacher told us that Columbus discovered America in 1492.
 
  用于现在完成时的句型 
  1）It is the first / second time…. that…结构中的从句部分，用现在完成时。
    It is the first time that I have visited the city.
    It was the third time that the boy had been late.
  2）This is the… that…结构，that 从句要用现在完成时.
    This is the best film that I've (ever) seen. 
      这是我看过的最好的电影。
    This is the first time (that) I've heard him sing.　这是我第一次听他唱歌。
典型例题
  (1) ---Do you know our town at all?　
    ---No, this is the first time I ___ here.
  A. was  B. have been  C. came  D. am coming
    答案B. This is the first time 后面所加从句应为现在完成时，故选B。
  (2) ---Have you ____ been to our town before?
    ---No, it's the first time I ___ here.
  A. even, come  B. even, have come  C. ever, come  D. ever, have come
    答案D. ever意为曾经或无论何时，反意词为never，此两词常用于完成时。 This is the largest fish I have ever seen. It is / was the first time +that-clause 的句型中，从句要用完成时。
    注意：非延续性动词的否定形式可以与表示延续时间的状语连用。即动作不发生的状态是可以持续的。
昨天已成过去
  （错）I have received his letter for a month.
  （对）I haven't received his letter for almost a month.
  比较since和for 
    Since 用来说明动作起始时间，for用来说明动作延续时间长度。
    I have lived here for more than twenty years.
    I have lived here since I was born..
    My aunt has worked in a clinic since 1949.　 
    Some new oilfields have been opened up since 1976. 
    I have known Xiao Li since she was a little girl.　 
    My brother has been in the Youth League for two years.　
    I have not heard from my uncle for a long time.
    注意：并非有for 作为时间状语的句子都用现在完成时。
    I worked here for more than twenty years. 
    　(我现在已不在这里工作。)
    I have worked here for many years.  　 
    　(现在我仍在这里工作。)
    小窍门：　当现在完成时+一段时间，这一结构中，我们用下面的公式转化，很容易就能排除非延续动词在完成时中的误使。
　1)　(对) Tom has studied Russian for three years.
  = Tom began to study Russian three years ago, and is still studying it now.
2)　(错) Harry has got married for six years.
　= Harry began to get married six years ago, and is still getting married now.
    显然，第二句不对，它应改为 Harry got married six years ago.　或 Harry has been married for six years. 
since的四种用法 
1) since +过去一个时间点　(如具体的年、月、日期、钟点、1980, last month, half past six)。
 I have been here since 1989.
2) since +一段时间+ ago
　I have been here since five months ago.
3) since +从句　Great changes have taken place since you left.
4) It is +一段时间+ since从句
　It is two years since I became a postgraduate student. 
延续动词与瞬间动词
  1) 用于完成时的区别
    延续动词表示经验、经历； 瞬间动词表示行为的结 果，不能与表示段的时间状语连用。
    He has completed the work.  他已完成了那项工作。 (表结果)
    I've known him since then.  我从那时起就认识他了。(表经历)
  2)　用于till / until从句的差异
    延续动词用于肯定句，表示"做……直到……" 瞬间动词用于否定句，表示"到……，才……"
    He didn't come back until ten o'clock.
      他到10 点才回来。
    He slept until ten o'clock.    　
      他一直睡到10点。
  　
  典型例题
  1. You don't need to describe her. I ___ her several times.　
  A. had met　B. have met  C. met　D. meet
    答案B. 首先本题后句强调对现在的影响，我知道她的模样，你不用描述。再次，several times告知为反复发生的动作，因此用现在完成时。

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