主語從句是整個主句的主語。它的位置一個是在整個主句之前，一個是在後面用it 做形式主語。如：What he said is true.(what he said是主語從句)。
It is certain that he will come.(that he will come是主語從句)。
而同位語從句是對一名詞或代詞的解釋説明。
它的位置在這一名詞或代詞之後，有的也被很短的謂語動詞隔開。
如 The news that he will come is true.(that he will come是同位語從句）。
Word came that we will have an examination.(that we will have an examination是同位語從句)。
