1. that，后跟陳述句，無意義，口語中可省略。

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

分析：從句she is an English teacher是一個完整的陳述句，不缺少任何成分(如主語、賓語等)，引導(dǎo)詞用that。

2. if和 whether表“是否”，多數(shù)情況下二者可互換。

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

※ 易錯點：只用whether不用if的情況

① 當與or not連用時

【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.

總結(jié)：無論or not位置在前面還是后面，出現(xiàn)or not就只用whether。

② 賓從作介詞的賓語時

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

介詞about后面，賓從的連接詞只能用whether。

③ Whether引導(dǎo)的賓從放在句首時

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

3. 特殊疑問詞。當賓語從句是特殊疑問句時，由疑問代詞(what、who、which、whose、whom等)和疑問副詞(where、when、why、how)引導(dǎo)。

※ 總結(jié)：問什么就用什么

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

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

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