I often hear people start a sentence with "But". I also find many books using a "but" at the start of a sentence. My teacher, however, told me that it's ungrammatical and that I'm not supposed to use "but" at the start of a sentence especially in formal documents.
My teacher says, for example, that:
A) Luffy was inspired to become a pirate. But his life changed forever when he ate the magical fruit.
should be
B) Luffy was inspired to become a pirate, but his life changed forever when he ate the magical fruit.
or
C) Luffy was inspired to become a pirate. However, his life changed forever when he ate the magical fruit.
Is it true that using "But" to start a sentence is ungrammatical and not accepted in formal documents?
