since是自從的意思\x0d一段時間 +since\x0d表示從這個時間開始到現(xiàn)在---自從~以來\x0d用法上It + ( 現(xiàn)在完成時 ) + 一段時間+ since + ( 一般過去時 )例句：\x0d1.It is / has been four days since I caught a cold.我感冒已四天了.\x0d2.It is / has been two weeks since we met last.自從我們上次見面以來已過了兩周.\x0d一段時間 + before\x0d表示在這個時間之前,或者大約在這個時間上就會發(fā)生某事\x0d用法上It + ( 一般將來時 ) + 一段時間+ before +( 一般現(xiàn)在時 )例句：It will be five years before he meets you again.\x0d五年之后他就會再見你的.\x0dIt + be (not) + long before\x0d表示"(不)很久 / 才…"或"(沒)過多久就 / …"\x0dIt was (not) long before ….(談?wù)撨^去)\x0dIt will (not) be long before ….(談?wù)搶?\x0dIt was long before she came back.\x0d沒過多久她就回來了.\x0dIt will be long before we meet again.\x0d很久以后我們才會再見面.\x0dIt will not be long before we turn our hope into reality.\x0d過不了多久我們的希望就會變成現(xiàn)實.\x0d這里it指時間,before是連詞,引導(dǎo)時間狀語從句.long可用一段時間來替代.