比較since和for

　　Since 用來說明動作起始時(shí)間，for用來說明動作延續(xù)時(shí)間長度。

　　I have lived here for more than twenty years.

　　I have lived here since I was born..

　　My aunt has worked in a clinic since 1949.

　　Some new oilfields have been opened up since 1976.

　　I have known Xiao Li since she was a little girl.

　　My brother has been in the Youth League for two years.

　　I have not heard from my uncle for a long time.

　　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。)

　　小竅門：當(dāng)現(xiàn)在完成時(shí)+一段時(shí)間，這一結(jié)構(gòu)中，我們用下面的公式轉(zhuǎn)化，很容易就能排除非延續(xù)動詞在完成時(shí)中的誤使。

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

　　since的四種用法

　　1) since +過去一個(gè)時(shí)間點(diǎn)　(如具體的年、月、日期、鐘點(diǎn)、1980, last month, half past six)。

　　I have been here since 1989.

　　2) since +一段時(shí)間+ ago

　　I have been here since five months ago.

　　3) since +從句

　　Great changes have taken place since you left.

　　Considerable time has elapsed since we have been here.

　　4) It is +一段時(shí)間+ since從句

　　It is two years since I became a postgraduate student.

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