The key to the whole affair was his jealousy. 整個事情的癥結是因為他忌妒。
Knowledge is a treasure, but practice is the key to it. [諺]知識是寶庫，實踐是鑰匙。
Do you know the key to the problem? 你知道這道題的答案嗎？
Seeing persons, however, do not need a metropolitan to find this key to beauty. 而能看見的人卻不需要到大都會博物館去找到這開啟美的鑰匙。
It would be with extreme reluctance that I should leave the Metropolitan Museum, which contains the key to beauty-- a beauty so neglected. 多麼不情願，我要離開大都會博物館，那裡有開啟美的鑰匙，這種美又被忽視了。
Here's your key to room218. 這是您218房間的鑰匙。
Reading is the key to the treasure house of ideas. 讀書是打開知識寶庫的鑰匙。
