"You must not drink."
Translation:Rhaid i chi beidio ag yfed.
It probably isn't. Like most other repetitive learning tools, Duo will bring in prompts from earlier sections to help you to practise and retain the earlier learning.
My answer was accepted without using the ag after beidio. I did use mae'n at the beginning though. Does that negate the need for a preposition later in the sentence? I thought those were always needed after a word like beidio