"Is it snowing?"
Translation:Ydy hi'n bwrw eira?
ydy at the start of a sentence is used to form a question (yw is not used for that). Mae is used to start non-emphatic statements.
This is explained in the 'Hints and Tips' for 'Present Tense 3', the section just before 'Weather'.
I thought it would be like this ('ydy' for questions), but the BBC big Welsh challenge actually have a scene where they say "Mae hi'n oer, tydy?". Does that work because it's a questioning statement or something?