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1. that，后跟陈述句，无意义，口语中可省略。

【例】They know(that)she is an English teacher.

分析：从句she is an English teacher是一个完整的陈述句，不缺少任何成分(如主语、宾语等)，引导词用that。

2. if和 whether表“是否”，多数情况下二者可互换。

【例】I wonder if/ whether you have already finished your homework.

※ 易错点：只用whether不用if的情况

① 当与or not连用时

【例】I wonder if/ whether you have already finished your homework.

= I wonder whether you have already finished your homework or not.

= I wonder whether or not you have already finished your homework.

总结：无论or not位置在前面还是后面，出现or not就只用whether。

② 宾从作介词的宾语时

【例】He was worried about whether he would lose his job.

介词about后面，宾从的连接词只能用whether。

③ Whether引导的宾从放在句首时

【例】Whether we can arrive there on time, I am not sure.

3. 特殊疑问词。当宾语从句是特殊疑问句时，由疑问代词(what、who、which、whose、whom等)和疑问副词(where、when、why、how)引导。

※ 总结：问什么就用什么

【例】I didn’t know who was late for school this morning. (问人——who)

【例】I don’t remember when we arrived here. (问时间——when)