"I think that you are pretty."
Translation:Dw i'n meddwl fod ti'n bert.
Dw i'n meddwl eich bod chi'n bert - I would say. Though colloquially sometimes the "eich" disappears, or is shortened to 'ch.
According to the tips and notes on this course the "eich" is rarely used, just as "dy" is not used in this example sentence.
Well as a fluent Welsh speaker, living in Wales and using the language every day, I would tend to at least make a nod to the presence of eich, with a 'ch at least. I would tend to drop the "dy" in "dy fod ti". But it is useful for a learner to learn it anyway, because without it, the mutation of bod=>fod would be difficult to understand.
I don't doubt that you're right, but it is slightly different from what the course teaches.
Given that "ti" is familiar and "chi" is polite/unfamiliar, I imagine calling someone you just met or is your superior pretty might not be a good idea.
Dim yn hollol siwr o fy ngramadeg, ond meddwl mae 'dy fod di' yn iawn, a 'dy fod ti' yn anghywir