if和when既可以引導(dǎo)賓語從句，也可以引導(dǎo)狀語從句，應(yīng)注意它們在兩種從句中的意思和用法的不同。if和when引導(dǎo)賓語從句時，分別意為“是否”和“何時”，其時態(tài)應(yīng)和主句時態(tài)相呼應(yīng);它們引導(dǎo)狀語從句時，意思分別為“如果，假如”和“當(dāng)……時候”，當(dāng)主句時態(tài)是一般將來時時，其時態(tài)用一般現(xiàn)在時。它們常常放在含有狀語從句和賓語從句的題干中進(jìn)行綜合考查。如：

　　—Do you know when he will come back tomorrow ?你知道他明天什么時候回來嗎?

　　—Sorry, I don't know. When he comes back, I'll tell you. 對不起，不知道。當(dāng)他回來了，我將告訴你。

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

　　—He will come if it doesn't rain.如果不下雨，他會來的。

　　★賓語從句的復(fù)合句轉(zhuǎn)化為簡單句(六種方法)，使其與原句意思相同(或相近)的試題。

　　方法：當(dāng)主句的謂語動詞是order(命令)，require(需要)等時，如果主句和從句的主語不一致，賓語從句可簡化為“名詞(代詞)+不定式”結(jié)構(gòu)。例如：The headmaster ordered that we should start at once. → The headmaster ordered us to start at once.

　　方法：某些動詞后的賓語從句，可以用介詞加動名詞(短語)等其他形式簡化。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：某些動詞后面的賓語從句可轉(zhuǎn)化為“賓語+V-ing形式(作賓語補足語)”結(jié)構(gòu)。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：當(dāng)主句謂語動詞是hope， decide， wish， choose， agree， promise(和主觀決定有關(guān))等，且賓語從句的主語與主句主語一致時，賓語從句可簡化為不定式結(jié)構(gòu)。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：當(dāng)主句謂語動詞是know， learn， remember， forget， tell(和記憶認(rèn)知有關(guān))等動詞，且主句主語與從句主語一致時，賓語從句可簡化為“疑問詞+不定式”結(jié)構(gòu)。例如：

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

　　注：當(dāng)主句謂語動詞是tell， ask， show， teach等動詞，且后帶雙賓語，從句主語和間接賓語一致時，賓語從句可簡化為“疑問詞+不定式”結(jié)構(gòu)。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：動詞seem后的賓語從句，也可以用不定式(短語)來簡化，但句型需要進(jìn)行適當(dāng)?shù)淖兓�。例如：It seemed that the boys were going to win. →The boys seemed to win.

　　除上述方法外，還有一些特殊句式的轉(zhuǎn)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