9判断一个从句是不是同位语从句的主要依据：（假设法）
假设that 引导的为定语从句，看that 在从句中是否担当成份，如：he father broke his promise (that) he made last week. 定语从句 that 担当了made的宾语，即made his promise,所以本句为定语从句，否则如The father made a promise that he will buy his son a new car.同位语从句，that不担当从句的成份，所以不是定语从句，为同位语从句。
The father made a promise that he will buy his son a new car.同位语从句
The father broke his promise (that) he made last week. 定语从句
下面这些词组（名词性的）常常跟同位语从句的名词:
(只是作为辅助方法判断同位语从句，不是主要依据。)
Advice， belief, doubt, explanation, fact, fear, feeling, hope,
Idea, news, opinion, order, possibility, promise, problem, probability
Question, reply, report, suggestion, thought, truth, wish, warning
其实，同位语从句的内容即是先行词的内容的具体解释或说明，中间几乎可以划等号。定语从句是对先行词的修饰、限定，不是先行词的内容本身。这个才是它们之间本质上的区别。
,
