一是作介詞,後面接名詞、動名詞、代詞、一般疑問句和特殊疑問句作賓語,可表示目的或原因,例如:Researchers have proposed several explanations for why interruption of glucose processing might retard aging
二是作引導詞,後面接陳述句,引導原因狀因從句,表示“因為,原因在於”,例如:They are less fully  given back to the water,for they still lay eggs on the beaches.