before引导的从句,主句用什么时态主句的动词是延续的吗它和when引导的什么区别

Question

before引导的从句,主句用什么时态 主句的动词是延续的吗 它和when引导的什么区别

Answer 1

首先你肯定是把before与since以及when混淆了.
it +现在完成时/一般现在时+时间段 since 一般过去时
it is/has been three years since we met for the first time.
it +一般过去时 +时间点 when 一般过去时
it was 7 P.M when the news was begun to report.
it+一般将来时 +时间点 when 一般现在时（表示将来的动作）
it will be 3 A.M when the train leaves for beijing.
下面两种句型都表示过了.才.
it+一般将来时 +时间段 before 一般现在时 （表示将来动作）
it will be three hours before we finish the work together.
it+ 一般过去时 +时间段 before 一般过去时
it was three years before the workers built the city museum.
同时掌握：
如果是从句中,明确的有当.某人正在.的意思时,主句用进行时态.
i was watching TV when my brother knocked at my door.
包含before的复合句中,before从句用一般过去时时,主句的谓语动作明显是发生在从句动作之前的,主句用“过去的过去”也就是过去完成时.
the train had left before/when she went to the station.