Since 用來說明動作起始時間，for用來說明動作延續時間長度。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。）

注意：用句型轉換的方法，很容易排除非延續動詞在有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.