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(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.
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