2015年衛(wèi)生類職稱英語考試語法知識：since/before引導的時間狀語從句

由since/before引導的時間狀語從句。

一般情況下，從句謂語動詞用一般過去時，而主句的謂語動詞用現在完成時。但在It is +時間+since從句的句型中，主句多用一般現在時。例如：

I have been in Beijing since you left.

It is four years since my sister lived in Beijing.(動詞延續(xù)，譯為否定句)

It was a long time before I went to sleep again.

It was an hour before(＝until) the police arrived.

She had known something about it long before I told her.

Do it before you forget it.

It wasn’t long before they got married.