文档介绍：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,    e --- 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;