主语从句是整个主句的主语。它的位置一个是在整个主句之前，一个是在后面用it 做形式主语。如：

What he said is true.(what he said是主语从句)。

It is certain that he will come.(that he will come是主语从句)。

而同位语从句是对一名词或代词的解释说明。它的位置在这一名词或代词之后，有的也被很短的谓语动词隔开。如 The news that he will come is true.(that he will come是同位语从句）。

Word came that we will have an examination.(that we will have an examination是同位语从句)。