首先定語從句可以由關係代詞who，whom，whose，which，that和關係副詞when，where，why引導.
而賓語從句則由連詞that，whether，if，when，where，how，why，who等引導.鮮明的標誌一般來說，賓語從詞是跟在動詞或介詞之後，而定語從句是跟在名詞或代詞（等價於名詞）之後.例如：
賓從：Foriegn language teachers are interested in how children learn to speak their native language.
Do you know who will give us the lecture next week?
定從：
Is is the photo that you took yesterday?
The thunder which was drawing nearer and nearer surprised the farm workers.
