比較since和for
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：并非有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.
（我現在已不在這里工作。）
I have worked here for many years.
（現在我仍在這里工作。）

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.