Reason 後接介系詞 for (不是 of) 再接名詞或動名詞片語，意為「某事的原因或做某事的原因」。例如：
- I don’t know the reason for her bad mood. (我不知道她心情不好的原因) (正)
I don’t know the reason of her bad mood. (誤)
- There were several reasons for his cruelty to animals. (他虐待動物是出於幾個原因) (正)
There were several reasons of his cruelty to animals. (誤)
- Could you tell me your reason(s) for marrying such an ugly woman? (你能告訴我你娶這樣一個醜女人的原因嗎？) (正)
Could you tell me your reason(s) of marrying such an ugly woman? (誤)
Reason 後面可以使用 of 的情況只出現在 by reason of (因為或由於某事) 和 for reasons of (基於…原因) 這兩個正式場合使用的片語中 (請注意這兩個片語中 reason 的單複數)。例如：
- He escaped caning by reason of his youth. (他因為年輕，得以免除鞭刑)
- The government has taken these measures for reasons of economy/national security. (政府採取這些措施是出於經濟上的原因/基於國家安全考量)
Reason 後面可接 why 所引導的子句，但 why 經常省略，尤其是在直述句中。例如：
- The reason (why) so many people had food poisoning is still not clear. (這麼多人食物中毒的原因仍然不明)
- The reason (why) Benson didn’t contact you was that he was only in Taipei for a few hours. (班森沒有跟你聯絡的原因是，他在台北僅待了幾小時)
Reason 不可跟 because 連用。例如：
- Is there any reason why Mary’s usually late for class? (為什麼瑪麗上課經常遲到？) (正)
- Is there any reason because Mary’s usually late for class? (誤)
Reason 後面可接 that 所引導的子句，但 that 經常省略，尤其是在直述句中。Reason that 比 reason why 略微正式一些，但不若 reason why 來得常用。例如：
- The reason (that) these cars are so expensive is that they are completely built by hand. (這些汽車如此昂貴的原因是，它們完全手工打造)
- One reason that Wesley decided to move to Kaohsiung was that his mother didn’t get along well with his wife. (衛斯理決定搬到高雄的一個原因是，他母親和他太太婆媳不和)
然而，當 reason 為複數時，reasons 只能與 why 子句連用，不可跟 that 子句連用。例如：
- There are several reasons why she quit after only three months in the job. (她只工作三個月就辭職有幾個原因) (正)
- There are several reasons that she quit after only three months in the job. (誤)
Reason + 不定詞
- I see no reason to refuse the job offer. (我沒有任何理由拒絕這項聘用通知)
= I see no reason for refusing the job offer.
= I can’t see any reason to refuse the job offer.
= There is no reason for me to refuse the job offer.
－ see no reason / not see any reason (to do sth or for doing sth) 意為「沒有任何理由 (做某事)」。
- There was every reason to accept her invitation. (有充分理由接受她的邀請)
= There was every reason for accepting her invitation.
= There were very clear reasons to accept her invitation.
－ there is every reason (to do sth or for doing sth) 意為「有充分理由 (做某事)」。
- I had every reason to accept her invitation. (我有充分理由接受她的邀請)
= I had every reason for accepting her invitation.
- I had no reason to accept her invitation. (我沒有理由接受她的邀請)
- This result gives us all the more reason to learn English. (這項結果讓我們更有理由學英文了)
= This result gives us all the more reason for learning English.
－ all the more reason (to do sth or for doing sth) 意為「更有理由 (做某事)」。