比较since和for
 
　　Since 用来说明动作起始时间，for用来说明动作延续时间长度。
　　I have lived here for more than twenty years.
　　I have lived here since I was born..
　　My aunt has worked in a clinic since 1949.　
　　Some new oilfields have been opened up since 1976.
　　I have known Xiao Li since she was a little girl.　
　　My brother has been in the Youth League for two years.　
　　I have not heard from my uncle for a long time.

　　注意：并非有for 作为时间状语的句子都用现在完成时。
　　 I worked here for more than twenty years.
　　　(我现在已不在这里工作。)
　　 I have worked here for many years.　　　
　　　(现在我仍在这里工作。)

　　小窍门：　当现在完成时+一段时间，这一结构中，我们用下面的公式转化，很容易就能排除非延续动词在完成时中的误使。
　　1)　(对) Tom has studied Russian for three years.
　　　 = Tom began to study Russian three years ago, and is still studying it now.
　　2)　(错) Harry has got married for six years.
　　　 = Harry began to get married six years ago, and is still getting married now.
　　显然，第二句不对，它应改为 Harry got married six years ago.　或 Harry has been married for six years.

since的四种用法
 
　 1) since +过去一个时间点　(如具体的年、月、日期、钟点、1980, last month, half past six)。
　 I have been here since 1989.
　
　 2) since +一段时间+ ago
　 I have been here since five months ago.

　 3) since +从句　
　 Great changes have taken place since you left.
　 Considerable time has elapsed since we have been here.

　 4) It is +一段时间+ since从句
　 It is two years since I became a postgraduate student.

延续动词与瞬间动词
 
1) 用于完成时的区别
　　延续动词表示经验、经历； 瞬间动词表示行为的结 果，不能与表示段的时间状语连用。
　　 He has completed the work.　　他已完成了那项工作。 (表结果)
　　 I've known him since then.　　 我从那时起就认识他了。(表经历)

2)　用于till / until从句的差异
　　 延续动词用于肯定句，表示"做……直到……" 瞬间动词用于否定句，表示"到……，才……"
　　 He didn't come back until ten o'clock.
　　　　他到10 点才回来。
　　 He slept until ten o'clock.　　　　　
　　　　他一直睡到10点。
　
典型例题
1. You don't need to describe her. I ___ her several times.　
A. had met　B. have met　 C. met　D. meet
　 答案B. 首先本题后句强调对现在的影响，我知道她的模样，你不用描述。再次，several times告知为反复发生的动作，因此用现在完成时。

2.---I'm sorry to keep you waiting.
　---Oh, not at all. I ___ here only a few minutes.
A. have been　 B. had been　 C. was　D. will be
　 答案A. 等待的动作由过去开始，持续到现在，应用现在完成时。