Since 用來說明動(dòng)作起始時(shí)間，for用來說明動(dòng)作延續(xù)時(shí)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(shí)間狀語的句子都用現(xiàn)在完成時(shí)。

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(zhuǎn)換的方法，很容易排除非延續(xù)動(dòng)詞在有for/since結(jié)構(gòu)的完成時(shí)中的誤用。

1）（對(duì)） Tom has studied Russian for three years. = Tom began to study Russian three years ago， and is still studying it now.

2）（錯(cuò)） 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(duì)，它應(yīng)改為 Harry got married six years ago. 或 Harry has been married for six years.