现在完成时需注意的问题：
1 短暂性动词不能与一段时间连用
appear,begin,borrow,lend,buy,close,come,die,fall,find,finish,join,kill,leave,sell,stop等
例如：He has joined the army for five years. (错)
He has been in the army for five years. (对)
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。
(错)I have received his letter for a month.
(对)I haven't received his letter for almost a month.
常考单词转换方法：
come/go/arrive/get/reach/move------be in/at
open-----be open
die------be dead
close----be closed
become---be
begin/start-----be on
asleep----be asleep
end/finish-----be over
join the army----be in the army/ be a soldier
join the Party----be in the Party /be a Party member
leave-----be away (from)
borrow---keep
put on----wear
buy-----have
catch a cold-----have a cold
2不能和明确的过去时间连用