unless引导条件状语从句 等于if … not …。例如:He'll accept the job unless the salary is too low. ( = He'll accept the job if the salary is not too low.)
在用as if引导的方式状语从句及表语从句中,根据情况要使用虚拟语气 例如:He talks as if he knew all about it. 但有时也可用直陈语气。例如:It looks as if it is going to rain.
whether, if引导从句的用法区别 引导主语从句、表语从句或同位语从句时,用whether,不用if。例如:Whether they will go to the Great Wall is not known./ The question is whether we can finish the task on time./ The question whether we will take part in the physics contest has not been decided. whether可接不定式,而if则不可。例如:I haven't decided whether to leave or not. whether可作介词的宾语或置于句首表示强调,而if则不可。例如:Everything depends on whether we have enough money./ Whether he will come, I am not sure. whether和 if均可引导宾语从句, whether引导的宾语从句一般都是肯定句,if引导的宾语从句可以是肯定的,也可以是否定的(此时不能用whether),例如:Could you tell us whether/ if it rains in winter in Australia?/ I wonder if it doesn't rain. 引导宾语从句的whether和if常可与or not连用。连用时要注意or not的位置,它一般与 whether、if分开使用,有时它可与whether合起来使用,但不能与if合起来使用。例如:I don't know whether/ if they will come or not./ I don't know whether or not they will come. if可用来引导条件状语从句,译"如果",whether则不行。例如:If you work hard, you are sure to succeed.
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