當先行詞reason在定語從句中作主語或賓語時,不用why,例如:
What was the reason that/which drove you to be a teacher是什麼原因驅使你當老師的(本句中,先行詞reason在定語從句中作主語,關係代詞用了that或which,不能用why)
The reason that /which he explained to us is not reasonable at all. 他跟我們解釋的理由一點都不合理(本句中心,先行詞the reason在定語從句中作賓語,關係代詞用了that 或which,不能用why)