1、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)

2、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.

3、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。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.　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.

4、as作从属连词可引导多种状语从句。①as引导时间状语从句，意为“当…时”。例如：As (he was) a young man, he was a storekeeper and later a postmaster./ He sang as he worked. ②as引导方式状语从句，意为“象…一样”。例如：We must do as the Party teaches us.　③as引导原因状语从句。意为“由于”，例如：As you are tired, you had better rest.　④as引导让步状语从句。意为“虽然”、“尽管”Child as he is, he can do it well. ( = Although he is a child, he can do it well.) 另外，as做为关系代词还可以引导定语从句，如：I have the same book as you.