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：

　　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。

　　它们的特点是：1、疑问词有本身的词义;2、疑问词在从句中担当句子成分，如主语、宾语或状语;3、这种疑问词引导的从句一律用陈述语序，不能用疑问语序。

　　例文：Do you know whom they are looking for? 你知道他们在找谁吗?(宾语从句，陈述语序。不能是whom are they looking for?)

　　I don’t know who did it. 我不知道这是谁干的。(宾语从句。在从句中如果疑问词作主语，其陈述语序和疑问语序一致。)

　　She asked me where I had been. 她问我到哪儿去了。(宾语从句。陈述语序，不能是where had I been. 直接引语，间接引语。注意这句话的时态。)

　　Can you tell me when the train will arrive? 你能告诉我火车什么时候到吗?(宾语从句。陈述语序，不能是when will the train arrive. 直接引语，间接引语。)

　　I don’t know why he hasn’t come yet. 我不知道他为什么还没来。(宾语从句，陈述语序。不能是why hasn’t he come yet.)

　　He didn’t tell me what you were doing. 他没和我说你在干什么。(宾语从句，陈述语序。不能是what were you doing.)

　　What you have done might do harm t other people. 你所做过的事情有可能伤害别人。(主语从句。what 作从句的宾语。陈述语序，不能是what have you done. 如果用疑问语序，意思发生了变化，成了“你做了什么?”。)

　　I don’t know where he is now. 我不知道他现在在哪里。(宾语从句，陈述语序。不能是where is he now.)

　　Where he went for his weekend is not known. 不知道他去哪里过的周末。(主语从句，where 作从句的地点状语。陈述语序，不能是where did he go for his weekend.)

　　Whose fault this is is not important. 这是谁的过错并不重要。(主语从句，whose 作从句的定语。陈述语序，不能是whose fault is this.)

　　What I want to know is where he has gone for his weekend. 我想知道的是他到哪里度周末去了。(这句话包含两个名词性从句：what I want to know是主语从句，what 在从句中作宾语，从句用陈述语序，不能是What do I want to know. Where he has gone for his weekend 是表语从句，where 在从句中作状语，从句用陈述语序，不能是where has he gone for his weekend.)