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同位语从句属于名词性从句，相当于名词，一般用来解释前面的名词，是名词的具体内容，不缺成分的；前面的名词常常为，news，fact，possibility，word， idea, truth, hope, problem, information, wish,promise,answer, evidence, report, explanation, suggestion, conclusion等抽象名词，如：
The news that they had won the game soon spread over the whole school
The general gave the order that the soldiers should cross the river at once
定语从句是修饰和限制前面的名词或代词或句子，而且它必须却成分的，不是主语，宾语就是状语，也就是前面的名词在后面充当成分的。如：1，Water consists of hydrogen and oxygen, as is known to all.2， One of my colleagues who you are familiar with will come today.,
3. The general’s daughter whose name is Jane, gave me a sweet smile.4. We are all proud of the Great Wall, which is one of the man-made wonders of the world.

非限制性定语从句是先行词的附加说明，去掉了也不会影响主句的意思，它与主句之间通常用逗号分开，将非限定性定语从句放在句子中间，其前后都需要用逗号隔开。