I'll give you a key so that you can let yourself in.
我把钥匙给你，你可以自己开门进去。
The key is hanging on a string by the door.
钥匙拴在门边的带子上。
The need to communicate is a key characteristic of human society.
需要交流是人类社会最重要的一个特征。
Cost was a key element in our decision.
价钱是我们决策时考虑的主要因素。
Good communication is key to our success.
良好的沟通是我们成功的关键。