在主从复合句中，从句可以充当主句的主语、表语、宾语或同位语。由于在多数情况下，主语、表语、宾语或同位语这四种句子成分由名词性词类充当，所以，我们把这些作用相当于名词的从句统称为名词性从句，把充当主语、表语、宾语或同位语的从句分别称为主语从句、表语从句、宾语从句或同位语从句。也就是说充当什么成分就叫什么从句。名词性从句由连接词(或关联词)引导。
常用的连接词有
whether, that(无词义)只起连接词作用，引导从句，在从句中不作任何成分，在从句中分别作主语、宾语和定语
who, whom, whose，which, what在从句中分别作主语、宾语和定语，除了起连接词作用外，还在从句中作状语
when, where, how, why疑问词(who, whom, whose, what, which, where, why, when, how)可以引导主语、宾语和表语从句。
名词性从句例句
1.That the seas are being overfished has been known for years.
海洋正在被人们过渡捕捞，许多年来这已是尽人皆知的事情了。
2.I have learned that love, not time, heals all wounds.
我明白了是爱，而不是时间能治愈一切创伤。
3.The reason for my return is that I left my keys behind.
我又回来，是因为我忘带钥匙了。
4.An idea came to her that she might do the experiment in another way.
她突然想到，她可以换个方法做实验。
