These words all describe sb who thinks and behaves carefully and sensibly, but often without much joy or laughter.以上各词均形容人严肃、稳重、认真。
serious thinking about things in a careful and sensible way; not laughing about sth指严肃的、稳重的、认真的：◆He's not really a very serious person.他并不真是一个非常严肃的人。◆Be serious for a moment; this is important.严肃点儿，这件事很重要。
grave(ratherformal) (of a person) serious in manner, as if sth sad, important, or worrying has just happened指人严肃、庄严、沉重：◆He looked very grave as he entered the room.他进屋时表情非常严肃。
earnest serious and sincere指认真的、诚实的、真诚的：◆The earnest young doctor answered all our questions.这个认真的年轻医生回答了我们所有的问题。
solemn looking or sounding very serious, without smiling; done or said in a very serious and sincere way指表情严肃的、冷峻的、庄严的、郑重的：◆Her expression grew solemn.她的表情显得严肃起来。◆I made a solemn promise that I would return.我郑重承诺过我会回来的。