He took a deep breath, desperately trying to keep calm.
他深深地吸了口气，竭尽全力保持镇定。
Try to keep calm and just tell me what happened
镇静点，告诉我出了什么事。
She has done her best to keep calm but finds herself trembling with tension and indecision
她尽力保持镇定，可还是由于紧张、犹豫不决而抖个不停。
Now that the situation becomes critical, one must keep calm.
事到临头，要沉住气。
Keep calm when problems crop up.
遇事冷静。
Keep calm! Don't panic!
沉住气，别慌！