since 用來說明動作起始時間，for用來說明動作延續(xù)時間長度。
　　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 作為時間狀語的句子都用現(xiàn)在完成時。
　　 i worked here for more than twenty years.
　　　(我現(xiàn)在已不在這里工作。)
　　 i have worked here for many years.　　　
　　　(現(xiàn)在我仍在這里工作。)

　　小竅門：　當現(xiàn)在完成時+一段時間，這一結(jié)構(gòu)中，我們用下面的公式轉(zhuǎn)化，很容易就能排除非延續(xù)動詞在完成時中的誤使。
　　1)　(對) tom has studied russian for three years.
　　　 = tom began to study russian three years ago, and is still studying it now.
　　2)　(錯) harry has got married for six years.
　　　 = harry began to get married six years ago, and is still getting married now.
　　顯然，第二句不對，它應改為 harry got married six years ago.　或 harry has been married for six years.