1、once作副詞譯“曾經”，作為連詞譯“一旦”，引導條件狀語從句。相當于if的加強形式。例如：I don’t believe he was once a thief. (once這里是副詞)/ 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連詞)

2、在用as if引導的方式狀語從句及表語從句中，根據情況要使用虛擬語氣。例如：He talks as if he knew all about it. 但有時也可用直陳語氣。例如：It looks as if it is going to rain.

3、whether, if引導從句的用法區別：①引導主語從句、表語從句或同位語從句時，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則不可。例如：I haven’t decided whether to leave or not. ③whether可作介詞的賓語或置于句首表示強調，而if則不可。例如：Everything depends on whether we have enough money./ Whether he will come, I am not sure. ④whether和if均可引導賓語從句， whether引導的賓語從句一般都是肯定句，if引導的賓語從句可以是肯定的，也可以是否定的（此時不能用whether），例如：Could you tell us whether/ if it rains in winter in Australia?/ I wonder if it doesn’t rain. ⑤引導賓語從句的whether和if常可與or not連用。連用時要注意or not的位置，它一般與 whether、if分開使用，有時它可與whether合起來使用，但不能與if合起來使用。例如：I don’t know whether/ if they will come or not./ I don’t know whether or not they will come.　⑥if可用來引導條件狀語從句，譯“如果”，whether則不行。例如：If you work hard, you are sure to succeed.

4、as作從屬連詞可引導多種狀語從句。①as引導時間狀語從句，意為“當…時”。例如：As (he was) a young man, he was a storekeeper and later a postmaster./ He sang as he worked. ②as引導方式狀語從句，意為“象…一樣”。例如：We must do as the Party teaches us.　③as引導原因狀語從句。意為“由于”，例如：As you are tired, you had better rest.　④as引導讓步狀語從句。意為“雖然”、“盡管”Child as he is, he can do it well. ( = Although he is a child, he can do it well.) 另外，as做為關系代詞還可以引導定語從句，如：I have the same book as you.