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常见的用法有：

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