He took a deep breath, desperately trying to keep calm.

　　他深深地吸了口氣，竭盡全力保持鎮定。

　　Try to keep calm and just tell me what happened

　　鎮靜點，告訴我出了什么事。

　　She has done her best to keep calm but finds herself trembling with tension and indecision

　　她盡力保持鎮定，可還是由于緊張、猶豫不決而抖個不停。

　　Now that the situation becomes critical, one must keep calm.

　　事到臨頭，要沉住氣。

　　Keep calm when problems crop up.

　　遇事冷靜。

　　Keep calm! Don't panic!

　　沉住氣，別慌！