"They have bad teeth" 可以用来表示他们的牙齿不好。这句话的来源可能是由于他们缺乏牙齿保健知识或缺乏口腔清洁习惯，导致牙齿出现问题，或者是由于遗传因素导致的。
下面是一些其他英文例句及其中文翻译：
"He has poor dental hygiene and has developed cavities." (他口腔卫生差，牙龈出现蛀牙。)
"Her teeth are yellow and discolored due to years of smoking." (由于多年吸烟，她的牙齿变黄变色。)
"Their gums are inflamed and bleeding because they don't brush their teeth regularly." (由于不经常刷牙，他们的牙龈发红流血。)