Other forms: buts
But is the word that turns a corner in a sentence. If a sentence starts out in one direction, but can change that direction. For example: "I like you, but I never want to see you again."
But can mean "however," "nonetheless," and "on the contrary" in three little letters. It's English efficiency at its best. But is short but powerful. But is everywhere but never unnecessary. But is never supposed to start a sentence, but sometimes it does. But sometimes means "just," as in: "I'll have but a few of those delicious candies." Grade schoolers might snicker, but the word would need another t to mean your "bottom."