摘要：Teenagers should not be sent to prison to mix with hardened criminals (=criminals who have committed and will continue to commit a
wanted criminal是警方搜捕的罪犯，大体相当于“通缉犯”，如
【例句1】He was one of the ten most wanted criminals in America. （他是美国十大通缉犯之一。） [剑桥商业英语词典]
violent criminal是“暴力罪犯”， 如
【例句2】Police sometimes put themselves in danger when arresting violent criminals. （警察在逮捕暴力罪犯时有时会把自己置于危险之中。 ）[牛津高级学者辞典]
【例句3】Police have described the man as a violent and dangerous criminal. 警方称该男子是一名暴力和危险的罪犯。)[朗文当代英语词典]
hardened criminal的含义是“惯犯”，如
【例句4】Society does not know how to deal with hardened criminals (= people who regularly commit crimes and are not sorry for what they do). (社会不知道如何对付惯犯)[牛津高级学者辞典]
“惯犯”也用habitual criminal，如
【例句5】Teenagers should not be sent to prison to mix with hardened criminals (=criminals who have committed and will continue to commit a lot of crimes).（青少年不应该被送进监狱，与惯犯混在一起。） [朗文当代英语词典]
【例句6】The new law will ensure that habitual criminals (=criminals who commit crimes repeatedly) receive tougher punishments than first-time offenders. （新法律将确保惯犯受到比初犯更严厉的惩罚。）[朗文当代英语词典]
criminal看起来像名词，也确实是形容词，但早在1620年代已有名词用法。品读例句，
【例句7】A group of gunmen attacked a prison and set free nine criminals in Moroto. （一群持枪歹徒袭击了莫罗托的一所监狱，释放了9名罪犯。 ） [柯林斯高级学者辞典]
【例句8】It should be a criminal offence to inflict cruelty on any wild animal. (虐待任何野生动物都应该是刑事犯罪。 )[牛津高级学者辞典]
【例句9】Criminal activities pose significant threats to the economy and national security. （犯罪活动对经济和国家安全构成重大威胁。）[牛津高级学者辞典]
【例句10】He admits to being a petty criminal(= who commits crimes that are not very serious). （他承认自己是个轻度罪犯。）[牛津高级学者辞典]
来源：秀方教育