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中考专题：状语从句
works hard.
’s in need of money.
He works hard because he’s in need of ，因n→be in, die→be dead, buy→have, arrive→be here等。'
自…以来 到现在
表示自过去的一个起点时间到目前(说话时间)为止的一段持续时间。主句一般用现在完成时，从句用一般过去时。但在It is +时间+since从句的句型中，主句多用一般现在时。
It is four years since my sister lived in Beijing. 我妹妹不在北京住有四年了。
Mr Green has taught in that school since he came to China three years ago.
自格林先生来中国以来，他就在这所学校教书。
2. by the time 到…为止 (所在句子的主句应用现在完成时或过去完成时)
By the time he gets there , his father has already gone.
他到家的时候，他爸爸已经走了。
By the time I got to school, the class had already began.
我到校时，已经开始上课了。
A, when, while和as的区别
when引导的从句的谓语动词可以是延续性的动词，又可以是瞬时动词。并且when有时表示"就在那时"。
When she came in, I was ，我在吃饭。(瞬时动词)
When I lived in the countryside, I used to carry some water for ，我常常为他担水。(延续性的动词)
We were about to leave when he came ，就在那时他进来了。
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While引导的从句的谓语动作必须是延续性的，并强调主句和从句的动作同时发生(或者相对应)。并且while有时还可以表示对比。例如:
While my wife was reading the newspaper, I was watching TV. (was reading是延续性的动词，was reading和was watching同时发生)
I like playing football while you like playing ，而你喜欢打篮球。(对比)
As表示"一边…一边"，as引导的动作是延续性的动作，一般用于主句和从句动作同时发生;as也可以强调"一先一后。
We always sing as we 。(as表示"一边…一边")
As we was going out, it began to ，开始下雪了。(as强调句中两个动作紧接着先后发生，而不强调开始下雪的特定时间)
，只能用when 引导这个从句，不可用as 或 while。
When you have finished your work, you may have a rest.
3、状语从句与主句的时态问题
、while引导的时间状语从句的时态： （主过从过）
主句（过去进行时）+when +从句（一般过去）
主句（一般过去时）+when +从句（过去进行时）
主句（一般过去时）+while +从句（过去进行时）
主句（过去进行时）+while +从句（过去进行时）
注意：看动词为短暂性动词用一般过去时，动词为延续性动词则用过去进行时。
小试牛刀：Mother （cook）supper when I (get) home yesterday.
Last Sunday I (meet) Lin Hong when I (walk) in the street.
When the students (have ) a meeting, the teacher (come) in.
While Han Meimei (sweep) the floor,