I've noticed that there are multiple forms of the "do" verb, such as: 'Rwyt ti' vs 'wyt ti', 'mae e' vs 'ydy e', 'dw i' vs 'ydw i', 'dych chi' vs 'dach chi'. Could someone explain me the difference? I do know that there are multiple levels of colloquiality in welsh, but apart from that - is there any other difference?
Also, it'd be great if the course included some explanation of these, instead of just popping them out now and then.
The verb is actually "to be" (bod). There is information on it here: https://www.cs.cf.ac.uk/fun/welsh/Lesson02.html or here: https://en.wikibooks.org/wiki/Welsh/Grammar/Verbs/Present (and no doubt many other places). In short: like every language I know, "to be" is quite irregular in Welsh. Add in colloquial forms and it looks daunting but don't panic - it is not really all that bad.
Rwyt ti=You (singular, familiar) are; wyt ti= are you?; dwyt ti ddim= you are not.
So R is an affirmative marker (it comes from "yr", "the" originally). D is a negative marker (it comes from "nid", "not" originally) and no marker makes it the question form.
Only those colloquial forms go and ruin the logic because "dw i" is short for "rydw i", "dych chi" is short for "rydych chi", etc. This only really happens in the present tense [edit: I mean the tenses using the present to form them, so all the "rydw" ones but not other tenses] so once you are used to them you won't think anything of it.
Also you mention mae e, and ydy e. That one is quite irregular, so:
mae e=he is; ydy e=is he?; dydy e ddim=he is not.
Great explanation, thanks. I also took a glance at the cs.cf.ac.uk page you referenced. Nicely written and humorous. I particularly enjoyed:
'The rule of thumb is that you use "ti" when talking to friends whom you know well (peers), children, animals (except maybe those bigger than you that you don't want to offend, like that bull over there), and Deity.'
But, I couldn't help laughing out loud at:
'Failure to use the formal form when you should could make you appear to be pushy or American or both;'!
On the one hand I like that English doesn't have politeness levels in its language because it makes it easier to speak it and not have to worry about abiding by hidden social rules. On the other hand, it makes it hard to learn other languages since every other language kept theirs!