I'll give you a key so that you can let yourself in.

　　我把鑰匙給你，你可以自己開門進去。

　　The key is hanging on a string by the door.

　　鑰匙拴在門邊的帶子上。

　　The need to communicate is a key characteristic of human society.

　　需要交流是人類社會最重要的一個特征。

　　Cost was a key element in our decision.

　　價錢是我們決策時考慮的主要因素。

　　Good communication is key to our success.

　　良好的溝通是我們成功的關鍵。