Rwyt ti and Wyt ti
I have done some of the Say Something in Welsh course, as I know many people here have (southern version) and this introduces 'wyt ti' for 'you are'. I've not come across 'rwyt ti' at all in those lessons (so far). So for me my automatic response is to use wyt ti.
Are there different occasions that you would use rwyt vs wyt? Is wyt just used more in spoken Welsh for example?
I guess it is the same with dach and dych - when do you use which? Is one north and one south?
Also, is chi formal you or just plural you?
There is a full list in the notes for Wanting2 in the course. As others have commented in speech people shorten words and the meaning is usually conveyed using context and intonation. So for example -you are going now- is 'rwyt ti'n mynd nawr' while the question is 'wyt ti'n mynd nawr?', you can use the same sentence in speech with different intonation, or even shorten to 'ti'n mynd nawr'.
Chi is formal you and also plural. For learners doing the first few units chi is easier to use.
Personally, I've always used Wyt ti for Are you, and Rwyt ti for You are. I have no idea how correct that is, someone fluent might want to completely correct me on that.
Yup, you're right. Dach chi is more Northern, and Dych chi is more Southern. In case you ever come across them, Rydych/Ydych mean the same thing too.
Chi is formal and plural you. Ti is single/familiar you; and since Chdi comes up on Duolingo sometimes too, that means You too.
Actually, on the topic of Say Something in Welsh, how are you finding it? I've tried doing it, but it seems a bit slow for my tastes (and I miss seeing the spellings/hearing the grammar), but would love to know how it is when you get further in!
To answer your questions about Say Something in Welsh, here are my impressions:
The Duolingo course is fairly broad, but shallow. By that I mean that you get a couple thousand words, and you get exposed to most of the important grammatical concepts, but it's going to take a lot of repetition and a lot of outside work to get those deeply into your head to the point where you can speak and listen naturally.
The SSiW courses, in contrast, are very narrow, but also very deep. The vocab is intentionally constrained to a smaller set. The grammatical concepts are not all fully fleshed out from the beginning. This can lead to frustration, but in my experience the stuff that you learn gets very deeply ingrained into you, which is what you need to be able to speak.
With that said, I'm 3/4 of the way through the Duo Welsh tree now, and I think that the only grammatical structure that I've encountered that I had not previously learned in SSiW is the passive voice. SSiW just takes a while to tie all the threads together.
I definitely find the two approaches complimentary and would recommend both to anyone studying yr hen iaith.
I don't know that there is a fixed rule. It just comes down to how it sounds. All of the 'y - yr' stuff, the contractions and shortenings, all happen to make things easier to say.
In this particular case, I think the way the 'r' is made with the tip of the tongue, followed by the 'h' in the back of the mouth feels very comfortable and natural.
Another SSiW'er here -- hi. I find the same thing you mention.
I think that 'rwyt ti' is technically the correct affirmative form, but the r gets dropped in speech much, or even most of the time. Just like 'roedd' vs 'oedd'. In both cases the 'r' is the modern remnant of the affirmative particle 'yr' that used to be used (and still is in literary welsh).
Just like the initial 'd' in the negative forms (doedd, dydy, dyw) is the remnant of the old negative particle 'nid'