Noun clauses refer to the clauses which are used as nouns in complex sentences. They include subject clauses, object clauses, predicative clauses, and appositive clauses. They are usually introduced by the following conjunctions.

从属连词 that, whether, if；
连接代词 who(ever), whom, whose, what(ever), which(ever)；
连接副词 when, where, why, how, how much / many /long /often 等。

主语从句在句中作主语
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.
It will take me 10 hours to get to my school.

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.
注意：
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：
T  he resistance of a wire also depends upon what material it is made of.
   We are sure (that) it will be a success.
3. 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.