for與since都能與現(xiàn)在完成時(shí)連用，但區(qū)別較大：
for后須接“時(shí)間段”，since后須接“時(shí)間點(diǎn)”，二者可以轉(zhuǎn)換，均須與延續(xù)性動(dòng)詞連用 初二，也可以與非延續(xù)性動(dòng)詞的否定式連用。二者引出的時(shí)間狀語往往用 how long提問。如：
He has been here for two years．＝He has been here since two years ago．他來這里已有兩年了。
—How long have you had this book？這本書你買多久了？
—For a week．／Since a week ago．買一周了。
We haven't heard from him for a month．我們已有一個(gè)月沒收到他的來信了。
since能引導(dǎo)時(shí)間狀語從句（主句用現(xiàn)在完成時(shí)，從句用一般過去時(shí)），而for則不能。如：
He has taught here since he came to China．自從他來到中國(guó)就在這兒教書。 It is ／has been an hour since he left here．他離開這兒已有一小時(shí)了。
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