for与since都能与现在完成时连用，但区别较大：
for后须接“时间段”，since后须接“时间点”，二者可以转换，均须与延续性动词连用，也可以与非延续性动词的否定式连用。二者引出的时间状语往往用 how long提问。如：
He has been here for two years．＝He has been here since two years ago．他来这里已有两年了。
—How long have you had this book？这本书你买多久了？
—For a week．／Since a week ago．买一周了。
We haven't heard from him for a month．我们已有一个月没收到他的来信了。
since能引导时间状语从句（主句用现在完成时，从句用一般过去时），而for则不能。如：
He has taught here since he came to China．自从他来到中国就在这儿教书。 It is ／has been an hour since he left here．他离开这儿已有一小时了。
