His parents have warned him about not working hard.

问题描述:

His parents have warned him about not working hard.
这句话中没有用到warn sb.not to do sth.是因为有about吗?

没有用到.
因为这句话意思是:他的父母在关于(他)不认真学习(的问题上)警告他.
warn sb.about在某事上警告某人
warn sb.not to do sth.警告某人别做某事(注意:不是这个人没有做这件事)