同位语从句属于名词性从句,大多由从属连词that引导,常常跟在fact,idea,opinion,news,hope,belief等名词后面.同位语从句一般用来解释或说明这些名词的具体含义或内容,在逻辑上表现为同位关系.例如：
They are familiar with the opinion that all matter consists of atoms.他们很熟悉这一观点,所有的物质都是由原子构成的.
1、从先行词来看
同位语从句与名词之间在逻辑上是"主语＋be＋表语"的关系.该名词是需要做特殊说明的抽象名词.例如：
They were delighted at the news that their team had won.当听到他们的球队赢了的消息时,他们欣喜若狂.
2、从引导词来看
引导词that在同位语从句中是连词,只起连接作用,无具体词义,that不可省略；that在定语从句中是关系代词,它在从句中充当一定成分---主语或宾语,有具体词意,作宾语时还可以省略.试比较：
The factory(that)we visited yesterday is a chemical one.我们昨天参观的那家工厂是化工厂.(that在从句中作宾语,所以此句是宾语从句)
The news that he will leave for Shanghai is true.他将要去上海的消息是真的.(that只起连接从句的作用,所以此句是同谓语从句)