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。
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：
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。)
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.
