"He is not laughing today."
Translation:Dydy e ddim yn chwerthin heddiw.
Why is "Mae e ddim yn chwerthin heddiw" not acceptable? That's what the hover option shows.
The hover hint shows dwy/dydy e ddim for 'he is not', and mae e... for 'he is' - you may need to look carefully to spot where the different word boundaries are.
See the course notes - https://www.duolingo.com/comment/17638579 - for an explanation of the the positive and negative forms.