这两种句型有很大的差别：
主语从句中的It是形式主语,真正的主句是that从句,所以that 从句+ be + 名词,这个句子的变化将会是通顺的.如：It is certain that he will win the match 可以理解为 that he will win the match is certain
同位语的作用相当于名词,对前面的名词（短语）加以补充说明或进一步解释,相当于一个表语从句,它们之间的关系是同位关系,即主表关系.如：I,the oldest girl in the family,always had to care for the other children 可以理解为 I am the oldest girl in the family
此外还要注意与强调句型 it is（was）.that 的区别,这种巨型是去掉it is（was）和that不会影响句子的意思,句子还是完整的.如：It was my parents that encouraged me to pursue my dream .