表語從句是對主語進行補充說明，是一種具體的描述，而IF 是一種假設或者判斷。

He looked just as he had looked ten years before.

他看起來還與十年前一樣.

表語是用來說明主語的身份、性質、品性、特徵和狀態的，表語常由名詞、形容詞、副詞、介詞短語、不定式、動詞的-ing、從句來充當，它常位於系動詞(be, become, appear, seem,look,sound,feel,get,smell等詞)之後。如果句子的表語也是由一個句子充當的，那麼這個充當表語的句子就叫做表語從句。表語從句(Predicative Clause)就是用一個句子作為表語。說明主語是什麼，由名詞、形容詞或相當於名詞或形容詞的詞或短語充當表語。

whether可以引導從句，作主語、表語或同位語，而if不能。如：

正： It was uncertain whether he would come.

誤： It was uncertain if he would come.

正： His first question was whether Tom had arrived yet.

誤： His first question was if Tom had arrived yet.

正： We must consider the question whether we will take these measures.

誤： We must consider the question if we will take these measures.