"Have you closed the cupboard?"

Translation: Dych chi wedi cau'r cwpwrdd?

[94BlueLane]

Why is it "wyt ti" instead of "rwyt ti" in this case?

[mizinamo]

Because it is a question, not a statement.

The r- comes from a particle yr that indicates a (positive) statement.

Questions had a particle a which got dropped entirely in speech.

And negative statements had a particle nid which turned into d- at the beginning.