since 用來說明動作起始時間，for用來說明動作延續時間長度。
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：並非有for 作為時間狀語的句子都用現在完成時。
i worked here for more than twenty years.
(我現在已不在這裡工作。)
i have worked here for many years.
(現在我仍在這裡工作。)
小竅門： 當現在完成時+一段時間，這一結構中，我們用下面的公式轉化，很容易就能排除非延續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.