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, leave, 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 have received his letter.

但是，不可以说：His father got ill for a week.

I have 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 haven’t heard from Jane for a long time.

I suppose something must have 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。