连接词
1)当由陈述句充当宾语从句时，用that引导，that无词义，在口语或非正式文体中常省略。如：
He said that he could finish his work before supper.他说他会在晚饭前完成工作。
2)当由一般疑问句充当宾语从句时，用if或whether引导，意为“是否”。如：
I don't know if /whether he still lives here after so many years.
但在下列情况下只能用whether：
在不定式前用whether。如：
He hasn't decided whether to visit the old man.他尚未决定是否拜访那位老人。
I don't know whether to go.我不知去否。
He hasn't decided whether to go by bus or by train.他还未决定是乘公共汽车去还是坐火车去。
whether置于句首时，不能换用if。如：
Whether this is true or not，I can't say.这是否真的我说不上来。
引导主语从句和表语从句时宜用whether。如：
Whether she will come or not is still a question.她是否能来还是个问题。
The question is whether we can catch the bus.问题是我们能否赶上公共汽车。
在具有选择意义，又有or或or not时，尤其是直接与or not连用时，往往用whether(if…or not也可以使用)。如：
Let me know whether /if he will come or not.(=Let me know whethe or not he will come)
I don't know whether/if he does any washing or not.(=I don't know whether or not he does any washing.)
I wonder whether we stay or whether we go.
在介词之后用whether。如：
I'm interested in whether he likes English.我关心的是他是否喜欢英语。
We're thinking about whether we can finish the work on time.
I worry about whether I hurt her feelings.我担心是否伤了她的感情。
若用if会引起歧义时，则用whether。如：
Please let me know if you like the book.可理解为：a. Please let me know whether you like the book.请告诉我你是否喜欢这本书。B. If you like the book, please let me know.你如果喜欢这本书，请告诉我。
3)如果宾语从句原来是特殊疑问句，只需用原来的特殊疑问词引导。如：
Could you tell me why you were late for the meeting this morning?
，我们将会及时处理。
