●主语从句在句中作主语
　　　　That mathematics finds its use in every science is evident.
　　　　Whoever is tired may rest.
　　　　Why the earth moves round the sun is quite clear now.

　　主语从句常放在句尾而用 it 作形式主语使句子更加平衡，这时口语中常省略 that, 如上面第一句改成“It is evident (that) mathematics finds its use in every science.”更好。

　　在下列情况下必须把主语从句放在句尾而用 it 作形式主语：
　　1． 主句谓语是被动语态时，如：
　　　　It is said that he is a good doctor.

　　2． 主句是疑问句或感叹句时，如：
　　　　Is it true that the film star will come?
　　　　How wonderful it is that we'll visit the Great Wall tomorrow.

　　3． 主句谓语动词是 appear, turn out, happen, occur, come, strike, follow, matter 等时，如：
　　　　It happened that she had a cold and didn't go with us that day.
　　　　It turned out that I was wrong.

　　4． 表示说话人的推测或评价，如在 it seems, it may be, it is (un)likely, it is (im)possible, it is a pity, it is a wonder 等结构中，如：
　　　　It seems that this test is reliable.
　　　　It is a pity (that) she can't go with us.

　　5． 强调主句的表语时，如：
　　　　It is a mystery to me how it all happened.
　　　　It is necessary that he should do so.

　　●宾语从句在句中作宾语
　　　　I thought (that) you had read the book.
　　　　We should learn to tell whether an element is poisonous or not.
　　　　Do you know who invented this?

　　1． 宾语从句后有宾语补足语时常移至句末，而用 it 做形式宾语放在前面，如：
　　　　The experiment makes it clear that air has pressure.

　　2 宾语从句可作某些介词和某些作表语用的形容词的宾语，如：
　　　　The resistance of a wire also depends upon what material it is made of.
　　　　We are sure (that) it will be a success.

　　3． 宾语从句可以是直接引语也可以是间接引语，如：
　　　　He asked me, “Are you from Chengdu?”（直接引语）
　　　　He asked me whether I was from Chengdu.（间接引语）

　　●表语从句在句中作表语
　　　　The reason is (that) we haven't raised enough money.
　　　　That is what we want to know.

　　●同位语从句在句中作补充说明名词及其短语的从句
　　　　I am in doubt whether I should buy or not.
　　　　He expressed his hope that he would visit China again.