2015年卫生类职称英语考试语法备考（3）
2015-02-06 11:06 来源：医学教育网
2015年卫生类职称英语考试语法知识：since/before引导的时间状语从句
由since/before引导的时间状语从句。
一般情况下，从句谓语动词用一般过去时，而主句的谓语动词用现在完成时。但在It is +时间+since从句的句型中，主句多用一般现在时。例如：
I have been in Beijing since you left.
It is four years since my sister lived in Beijing.(动词延续，译为否定句)
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.
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