Q. Is it grammatically correct to start a sentence with the word because?
A. Yes, it’s correct. It’s correct in formal prose when because is the beginning of a complete sentence, e.g.,
Because of the wind, it felt colder.
Because I was late, they towed my car.
Sticklers object to the use of because because it sometimes introduces a sentence fragment, and they think that sentence fragments are not allowed in writing. But they are wrong—sentence fragments are found in the very best of classic English prose. Because they work.