if和when既可以引导宾语从句，也可以引导状语从句，应注意它们在两种从句中的意思和用法的不同。if和when引导宾语从句时，分别意为“是否”和“何时”，其时态应和主句时态相呼应;它们引导状语从句时，意思分别为“如果，假如”和“当……时候”，当主句时态是一般将来时时，其时态用一般现在时。它们常常放在含有状语从句和宾语从句的题干中进行综合考查。如：

　　—Do you know when he will come back tomorrow ?你知道他明天什么时候回来吗?

　　—Sorry, I don't know. When he comes back, I'll tell you. 对不起，不知道。当他回来了，我将告诉你。

　　—I don't know if he will come.我不知道他是否会来。

　　—He will come if it doesn't rain.如果不下雨，他会来的。

　　★宾语从句的复合句转化为简单句(六种方法)，使其与原句意思相同(或相近)的试题。

　　方法：当主句的谓语动词是order(命令)，require(需要)等时，如果主句和从句的主语不一致，宾语从句可简化为“名词(代词)+不定式”结构。例如：The headmaster ordered that we should start at once. → The headmaster ordered us to start at once.

　　方法：某些动词后的宾语从句，可以用介词加动名词(短语)等其他形式简化。例如：

　　He insisted that he should go with us. →He insisted on going with us.

　　The poor boy doesn’t know when and where he was born. →The poor boy doesn’t know the time and the place of his birth.

　　方法：某些动词后面的宾语从句可转化为“宾语+V-ing形式(作宾语补足语)”结构。例如：Liu Ping found that there was a wallet lying on the ground. → Liu Ping found a wallet lying on the ground.

　　方法：当主句谓语动词是hope， decide， wish， choose， agree， promise(和主观决定有关)等，且宾语从句的主语与主句主语一致时，宾语从句可简化为不定式结构。例如：

　　Li Ming hopes he will be back very soon. →Li Ming hopes to be back very soon.

　　We decided that we would help him. →We decided to help him.

　　方法：当主句谓语动词是know， learn， remember， forget， tell(和记忆认知有关)等动词，且主句主语与从句主语一致时，宾语从句可简化为“疑问词+不定式”结构。例如：

　　She has forgotten how she can open the window. →She has forgotten how to open the window.

　　注：当主句谓语动词是tell， ask， show， teach等动词，且后带双宾语，从句主语和间接宾语一致时，宾语从句可简化为“疑问词+不定式”结构。例如：

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

　　方法：动词seem后的宾语从句，也可以用不定式(短语)来简化，但句型需要进行适当的变化。例如：It seemed that the boys were going to win. →The boys seemed to win.

　　除上述方法外，还有一些特殊句式的转化。例如：

　　I found that it was difficult to learn English well. →I found it difficult to learn English well.

　　Soon we found that the ground was covered with thick snow. →Soon we found the ground covered with thick snow.

　　They found that the box was very heavy. →They found the box very heavy