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.
