What is But ?
But is (conj) coming before a contrast He is very tall, but his wife is quite short. We would like to come to your party, but we’re doing something else that evening. I’m sorry, but there are no seats left. (prep) except Everyone but me is allowed to go to the cinema. They had eaten nothing but apples. (adv) only This is but one of his mistakes. We can but try.
source: Easier English, Student Dictionary Upper Intermediate Level