宾语从句是名词性从句的一种,在句子中起宾语作用.宾语从句分为三类：动词的宾语从句,介词的宾语从句和形容词的宾语从句.如：I am sorry that I have troubled you so long.很抱歉我这么长时间在打扰你.that I have troubled you so long部分为宾语从句,that引导宾语从句无意义,不充当句子成分.
而定语从句是形容词性从句,在句中做定语,修饰一个名词或代词,被修饰的名词、词组或代词即先行词.定语从句通常出现在先行词之后,由关系代词或关系副词引出.关系代词所代替的先行词是人或物的名词或代词,并在从句中充当主语、宾语、定语等成分.如：This is the book that you want.关系代词that在句中作动词want的宾语.而关系副词则在句中作状语,如：Shanghai is the city where I was born.关系代词where在从句中作地点状语,相当于介词in+which,把先行词city代入从句即：I was born in Shanghai.