严肃(yán sù) - serious in Mandarin

严肃 (yánsù) means "serious", describing a person's manner, expression or tone rather than the gravity of a situation (for that sense, 严重 yánzhòng, "serious, grave", is the word wanted instead, and the two are a classic mix up). 严 (yán) means "strict, severe", and 肃 (sù) means "solemn, respectful", together giving a stern, no nonsense air.

严肃 (yánsù) most often describes a face, a voice or an atmosphere: 严肃的表情 (yánsù de biǎoqíng), "a serious expression", and 气氛很严肃 (qìfēn hěn yánsù), "the atmosphere is very serious", are both natural. It is not the word for a serious illness or a serious accident, where 严重 (yánzhòng) takes over.

The antonym most often reached for is 幽默 (yōumò), "humorous", or 随和 (suíhe), "easy going", depending on whether the contrast is about humour or approachability.