6.once作副词译“曾经”,作为连词译“一旦”,引导条件状语从句。相当于if的加强形式。例如:I don’t believe he was once a thief. (once这里是副词)/ Once Aristotle had made up his mind that heavy objects always fell faster than light objects, he taught it as a truth to his students. (once连词)
7.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.)
8.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.
9.whether, if引导从句的用法区别:①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. ②whether可接不定式,而if则不可。例如:I haven’t decided whether to leave or not. ③whether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. ④whether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. ⑤引导宾语从句的whether和if常可与or not连用。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. ⑥if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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