"Do you want toothpaste?"
Translation:An dteastaíonn taos fiacla uait?
Ba mhaith leat … means “You would like …”, and is only used for a statement, not for a question. Its interrogative form would be Ar mhaith leat … ?.
Although they express the same request, “Would you like … ?” and Ar mhaith leat … ? are more polite than “Do you want … ?” and An dteastaíonn … uait? are, and so aren’t completely interchangeable in all situations.