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(zhuǎn)換的方法,很容易排除非延續(xù)動詞在有for/since結(jié)構(gòu)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(yīng)改為 Harry got married six years ago.或 Harry has been married for six years.