Why in this sentence "bwydo'r ci" the word "ci" is not softened? and in "glanhau'r ddesg" the word "desg" is softened?
As explained in the course notes, feminine nouns take a weak soft mutation after the definite article 'r/y. ci is a masculine noun, so does not mutate here.
(In a weak soft mutation, words beginning ll- or rh- resist the mutation.)
Is "wyt ti wedi bwydo'r ci?" an acceptable answer? Not sure if it's a mistake.
"Wyt ti wedi bwydo'r ci?" is accepted. It's the informal form you would use amongst friends etc.
Besides being formal, can "dych chi" when referring to plural? kind of like the difference between "tu" and "vous" in French?
Yes, that’s how it works.
Is bwydo a different verb form of bwyta? Or is it that they are related but not as directly as that?
so, "wyt ti..." and "dach chi..." is the same as "dych chi..."?
What are the differences between using 'dach' and 'dych' or are they completely interchangeable?
'Dach chi is the form used in North Wales, 'Dych chi is the form used in the South and is a more standard.
In written Welsh 'rydych chi' is the standard.
When is "Ydych chi" used? I don't remember learning this.
Ydych chi...? is a form for asking a question. In the colloquial language it is frequently shortened to Dych chi...? and that is the form most widely taught on Welsh for Adults courses.