If引导的条件状语从句（用法）

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状语从句：用一个句子（从句）来作另一个句子（主句）的状语,用作状语的句子就叫作状语从句.作什么样的状语就叫什么类型的状语从句.例如：条件状语从句就是用一个句子来作条件状语.状语从句的分类：（1）时间状语从句凡是从句都必须有引导词,引导时间状语从句的词有when,before,after,until,as soon as,while 等.（2）条件状语从句主要看一下由if引导的条件状语从句.if 意为“如果”,引导条件状语从句时,表示假如有从句的动作发生就（不）会有主句的动作发生.例如：If it doesn't rain tomorrow,we will go there by bike.如果明天不下雨,我们就骑自行车去那里.If I get there early,I can see the doctor quickly.如果我早早地到那里,我就可以快点看病.3）地点状语从句用法要点常用where（哪里）和wherever（无论哪里）eg.Where there is a will,there is a way.有志者,事竟成.We will go wherever the motherland need us most.我们要到祖国最需要的地方去.（4）原因状语从句用法要点常用的引导连词有because,as和since,三者的区别是：在回答问题的时候,使用because；对于显而易见的原因,常用as或since；as和since的从句常放在主句之前,而because的从句常放在主句之后.eg.Why did you go?I went because Tom told me to go.你为何去?那是因为汤姆叫我去.He was angry not because we were late but because we made a noise.他很生气不是因为我们来迟了,而是因为我们弄出了声音.As it was raining hard,we had to be indoors.由于雨太大,我们只好呆在家里.Since you feel ill,you'd better not go to work.既然你感觉不舒服,你最好不要去上班了.（5）目的状语从句用法要点常用的引导连词有so that,that和in order that译为：以便,为了,目的是.eg.Please speak more slowly so that we can make full notes.请讲慢一点,以便我们能作详细笔记.I shall write down your address that I may not forget.我要把你的地址记下来,以免忘记.I sent the letter by air mail in order that it might reach him in time.这封信我以航空信发出,以便他能及时收到.注：目的状语从句可以用动词不定式来替换做目的状语.eg.We work harder than usual finish it in a week.我们比平常加倍努力工作以在一周内完成工作.三、主句与从句时态一致的问题.在时间状语从句和条件状语从句中,主句和从句之间的时态一致问题一般分下列两种情况：1.若主句是祈使句,或主句中有情态动词,或主句中谓语动词是一般将来时,那么从句用一般现在时表示一般将来时.例如：Be careful when you cross the road.过马路时要小心.Put up your hand if you have any questions to ask.如果你们有问题要问的话,请举手.The traffic must stop when the lights are red.红灯亮时,车辆必须停下.2.若主从句谓语动词都是陈述过去,那么主从句都可以用一般过去时.例如：She could sing when she was four years old.她四岁的时候就会唱歌.