短暂性动词，延续性动词和现在完成时。
短暂性动词是指动作一旦发生就立即结束。它不能和一段时间连用。这类动词有：
go, come, arrive, le**e, begin, start, join, marry 等；短暂性动词的肯定式不能与表示一段时间的for或since或how long等状语连用。例如：
his father got ill. i h**e received his letter.
但是，不可以说：his father got ill for a week.
i h**e received his letter for three days.
在否定句中，短暂性动词可以和表示一段时间的状语连用。例如：
we h**en’t heard from jane for a long time.
i suppose something must h**e happened to her.
延续性动词是指动作发生后还可以延续一段时间或者相当长的一段时间。如：live, work, study, learn, sleep等，延续性动词可以和表示一段的时间状语连用。
值得注意的是，表示一段时间的状语除了“for+一段时间”外，还有since+n./从句（时间点），since+时间段+ago，疑问词how long，in the past+时间段，all these+时间段，from…to（时间点）以及till/until+n./句子等等。
例如：how long __your brother joined the army?
a. has b. had c. is it since d. was there that
通过审题，答案b和d很容易被排除，但是稍不注意便会选中答案a；殊不知how long为一段时间的状语，不能与非延续性动词短语has joined连用，所以这题的正确答案是c。
另外，动词的延续性与非延续性还体现在这两种句型中：
延续性动词的肯定式+till/ until 短暂性动词的否定式+till/ until
请看例句：we waited until he came.
i didn’t go to bed until my father came back home last night.
如果短暂性动词与表示一段时间的时间状语连用，必须将短暂性动词改为延续性动词。
现归纳总结一下由非延续性动词到延续性动词的转换：
arrive→be herebegin(start)→be on
die →be deadcome back→be back
le**e →be away (fromfall ill(sick, asleep)→be ill(sick, asleep)
get up→be upgo out →be out
finish →be overput on →wear 或be on
open →be openjoin →be in或 be a member of…
close →be closedgo to school→be a student
borrow →keepbuy/get →h**e
catch(a cold)→ h**e(a coldget to know →know
begin to study→studycome to work→work
move to → live infinish/end → be over
come to → be insit down → be seated
marry → be marrieddress → be dressed等。
如：he has been a soldier for three years. 他参军三年了。
his father has been dead for two years. 他父亲去世二年了。
the film has been on for ten minutes. 电影已开始十分钟了。
we h**e studied english for three years. 我们（开始）学英语已三年了。
how long can i keep this book? 这本书我可以借多长时间？
除了用上面的转化形式之外，还可以用其他的表达形式：a. it is/ has been+时间+since……（动词用过去式），b.
主语+谓语（过去时）+时间+ago。 例如：
it’s five days since i received his letter.
his father died three years ago.
看一个句子可以用以下几种说法：
他离开上海已经3天了。
he left shanghai three days ago.
he has been away from shanghai for three days.
he has been away from shanghai since three days ago.
it is/ has been three days since he left shanghai.
three days has passed since he left shanghai.
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