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.