首先看引导从句的关系词that是否在从句中担任句子成分。假如在从句中担任主、宾、表，则是定语从句。再看that前的表名称的词是否是一点需求有内部实质意义的表名称的词，如：idea, fact, thought, news等。(belief, dou变态, fact, hope, idea, news,possibility, thought, message, word, problem, promise, truth, information, plan, suggestion, opinion, problem, question, decision, answer, reply)后面的从句是解释明白其内部实质意义的，（这个表名称的词在后面的从句中部担任不论什么成分），这个从句则是同位语从句

 例1、The news that he had been back surprised us all. (that指导同位语从句)

    例2、He had to settle the problem that they left. (that指导定语从句)

区别两类从句的办法是看关系词是否在从句中担任成分，若担任成分的就是定语从句(如例2， that在从句中担任left的宾语)：不担任成分而只是论述面前表名称的词的具体内部实质意义的，就是同位语从句(如例1，he had been back就是news的具体内部实质意义)。

还是从语气上鉴别
从句位于少量几个表名称的词，如 suggestion，advice，order，request 等后时，同位语从句的谓语动词务必用虚拟语气，既 should ＋动词原型， should可以省略;而定语从句则没有这一要求。如：
The advice that we (should) build a school in that small village was supported by all of us. （同位语从句）
The advice that he gave was supported by all of us. （定语从句）