Since 用来说明动作起始时间,而for用来说明动作延续时间长度.
比如说 ：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.
但是也并非有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.)