if和 when既能引导时间状语从句，又能引导宾语从句。因此，遇到它们就要认真分析一下，它们究竟属于“两副面孔”的哪一副。如：

If it rains tomorrow， I won’ t come.(时间状语从句)

I don’ t know if it will rain tomorrow. (宾语从句)

从句中人称与标点的变化

1. 从句的主语如果是第一人称，变为宾语从句则与主句的主语一致;如果是第二人称，则与主句的宾语一致;如果是第三人称，不用变化。如：

“ May I use your knife?” He asked me. → He asked me if he might use my knife.

“ Do you know her telephone number?” He asked me. → He asked me if I know her telephone number.

2. 宾语从句的标点均由主句决定。主句是陈述句，用句号;主句是疑问句，用问号。如：

Who will give us a talk? I don’ t know. → I don’ t know who will give us a talk.

Do you know? Where does he live?→ Do you know where he lives?

宾语从句的简化

1. 当主句谓语动词是 find， see， watch， hear等感官动词时，从句常简化为“宾语+宾补”结构，宾补为不带 to的不定式或 V-ing形式。如：

She found that the wallet lay/ was lying on the ground. → She found the wallet lie/ lying on the ground.

2. 当主句谓语动词是 hope， wish， decide， forget， plan， agree等，且主句主语与从句主语相同时，从句可简化为不定式结构。如：

She agreed that she could help me with my maths. → She agreed to help me with my maths.

3. 在连接代词/副词引导的宾语从句中，当从句主语与主句主语或间接宾语一致时，宾语从句可简化为“连接代词/副词+不定式”结构。如：

Can you tell me how I can get to the station?→ Can you tell me how to get to the station?

宾主从句的时态变化

( 1)若主句是一般现在时、一般将来时或祈使句时，宾语从句根据句子的意义使用需要的时态。如：

We don’ t know who has broken the window. But we will find out who has done that.

Please tell me where we will go.

( 2)若主句是一般过去时，宾语从句要用相应的某种过去时态，如一般过去时、过去进行时、过去将来时或过去完成时。如：

He said: I’m a student. - - - - - He said that he was a student.

The teacher asked: What is Mike doing? - - - - - The teacher asked what Mike was doing.

( 3)若宾语从句叙述的是客观真理或永恒不变的规律，则应采用一般现在时，不受主句时态的限制。如： Our teacher said that the earth moves round the sun.