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.