分別舉個例子：
賓語從句：
i don't know what to do.i don't know which one is the best.象這種有what\which\that引導句子在整個句子充當賓語成分.那就是賓語從句.主語：i 謂語：don't know 賓語：what to do.
定語從句：
i don't know the man who is wearing a pair of glass.象這中由who,which,that,when 引導的句子在整個句子中做定語成分就叫定語從句.
主語：i
謂語：don't know
賓語：the man
定語：who is wearing a pair of glass.修飾the man.
狀語從句：
i don't know she is my english teacher when i see her at first time.我第一眼見到她的時候,不知道她是我的英語老師.
象這樣以when引導的句子在整個句子中充當狀語成分的就是狀語從句.
主語：i
謂語：don;t know
賓語：she is my english teacher.
狀語：when 引導的句子.