but在英语句子中可用作关系代词，引导定语从句。但应注意but虽然形式上是肯定的，意义上却是否定的。这正是but和其它关系代词(who, that , which等)不同之处。例如：
There is no rule in English but has exceptions.英语中没有无例外的规则。
There is no old habit but may be cured by a strong will-power.通过强大的意志力没有改不了的旧习惯。
There is no human being but errs.没有不犯错误的人。
如果将以上例句中的关系代词but改成that，定语从句必须改成否定从句才能与原句意义相一致。例如：
There is no rule in English that has no exceptions/ that has not any exceptions.
There is no old habit that may not be cured by a strong will-power.
There is no human being that(who)makes no mistakes
but的含义为：that/which /who
