but可被看作关系代词,引导定语从句,在从句中作主语,意义上相当于 who not或that not,即用在否定词或具有否定意义的词后,构成双重否定.如：
1）There is no mother but loves her children．没有不爱自己孩子的母亲.
2）There was no one present but knew the story already．在场的人都知道这个故事.