Although the meaning is pretty much the same, that's not exactly what's being asked, hence why you were probably marked wrong:
Beth fydd y tywydd yfory? "What will the weather be tomorrow?"
Sut bydd y tywydd yfory? "How will the weather be tomorrow? / What will the weather be like tomorrow?"
New patterns are explained in the course notes (https://forum.duolingo.com/comment/17638579). For example, see https://www.duolingo.com/skill/cy/Weather/tips-and-notes and https://www.duolingo.com/skill/cy/FutureBod/tips-and-notes