"Are you going to Aberteifi on Monday?"

Translation: Dych chi'n mynd i Aberteifi dydd Llun?

[kineticwab]

The English version should probably say "Cardigan", no? :)

[MatthewSke1]

Why is "Rwyt ti'n" wrong here?

[mizinamo]

Because this is a question, and "rwyt" is for the form used in statements.

"Wyt ti?" should be accepted, though.