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。例如：

　　I have lived here for more than twenty years.我住在這兒二十多年了。

　　I have lived here since I was born. 我從出生起就住在這兒了。

　　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。)

　　注意：用句型轉換的方法，很容易排除非延續(xù)動詞在有for/since結構的完成時中的誤用。

　　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.