@hund You can't start a sentence with ‘but’ because then it would not be a complete sentence (that is, if you isolate it from the rest of the discourse, it makes no sense by itself). A ‘but’ only works in opposition to something else, which is why it should be preceded by whatever it's opposing and separated from it by a comma or a semicolon.

Of course, when I say “you can't” I really mean “you shouldn't if you want to adhere strictly to a correct syntax.“ Informally, we all understand stuff like: “But I want candy, mom!”