No one answered the ad but me. In this case the word BUT is functioning as an adverb meaning EXCEPT. Usually, in this constructions the word BUT is preceded by no one, nobody, everyone, everybody, everthing, or nothing. There is but one choice to make in this case. In this case, the word BUT is also functioning as an adverb meaning MERELY or NOTHING BUT. Sadie said she would go but she had to bring her sister. In the last case, the word BUT is working as a conjunction or coordinator, and it's linking two clauses together to make a single sentence. Sadie said she would go it's the main clause and she had to bring her sister is the subordinate clause.