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1、条件状语从句条件状语从句在复合句中表示主句动作发生的条件。一.if引导的条件状语从句.用法条件状语从句最常用的引导词是if,意为“如果，假 如”，表示在某种条件下某事很可能发生。If it doesn t rain tomorrow , we will go hiking .You will get good grades if you study hard.if引导的条件状语从句可以位于主句之前，也可以位于主 句之后。如果从句在前，和主句之间要用逗号隔开。1 .时态表示将来可能发生的事情；主句：常用一般将来时。 从句：用一般现在时。（主将从现）If you ask him, he will
2、help you.If he doesn t study hard, he will not pass the exam.If it doesn t rain tomorrow, we will go hiking.They are going to have a picnic if they are free next Saturday.主句可含有情态动词或一个祈使句。从句：用一般现在时。If it stops snowing, we can go out.Don t wait for me if V m late .（3表示与现在事实相反的情况，主句： would/should/could
3、/might+动词原形。从句：一般过去 时，be动词常用were.If I had his telephone number, I would call him now.If I were you, I would not tell him that thing. 二.unless引导的条件状语从句。1 .用法条件状语从句也可由unless来引导，它的意思是“除 非，如果不”，表示某件事只在特定情形下才会发生或成 立。Unless you study hard, you will never pass the exam.You will be late unless you go at onc
4、e.2 .时态(1)主句：常用一般将来时。从句：用一般现在时。(主将 从现)Unless it rains, the game wil1 be played.You won t know the answer unless you listen carefully.(2)主句：可含有情态动词或一个祈使句。从句：用一般 现在时。He can t see clearly unless he wears glasses.You must stop if the traffic light is red.If you drive too fast, you may have an accident.D
5、on t make any promise unless you can keep it.Don t touch the machine unless the teacher allows you to.Don t watch TV if you don t finish your homework.注：在“祈使句+and/or+陈述句”结构中，祈使句在意义上 相当于条件状语从句，二者可以相互转换。Use your head, and you 11 find a way.=If you use your head, you 11 find a way.you won t know the an
6、swer if you don t listen carefully.=You won t know the answer unless you listen carefully.= Listen carefully, or you won t know the answer. 三.unless 与 if. . not 的区另限(1) if. . . not与unless 一样，也表示“除非.，如果 不.”，在很多情况下，二者可以互换使用。I won t go to the party if I m not invited .=I won t go to the party unless I
7、m invited.If it doesn t rain tomorrow, we will go hiking.=We will go hiking unless it rains tomorrow.She will keep on singing if she is not told to stop. =She will keep on singing unless she is told to stop.(2)当某种条件所引起的结果是一种情绪或想法时，只能用 if. . . not ,不用 unless .I will be happy if I don t have to do the
8、 homework. (4)I will be happy unless I have to do the homework, (x)I will be angry if I am not invited to the party. (Y)I will be angry unless I am invited to the party, (x)(3)当表示与现在事实相反的情况时，只能用if. not,不用 unless.主句：would/should/could/niight+动词原形。从句：一般 过去时，be动词常用were.If you weren t always in such a hurry ,your work would be much better. (4)Your work would be much better unless you were always in a hurry, (x)If he weren t so careless , he might pass the exam, (d)He might pass the exam unless he were so careless, (x)