What is the difference between "rwyt ti"and "wyt ti"?
Sorry question again.
In essence, they are the same - just mutated, depending on if it is used as a question or in the negative. [As MrGWallCymraeg points out, I was incorrect - it is not a mutation. See his great explanation below.]
Rwyt ti = You are.
Wyt ti = You are?/Are you?
Dwyt ti (ddim) = You are not.
(also, never be sorry for asking a question! that's how we learn :) )
It's not a mutation as such; "rwyt ti" comes from "yr wyt ti" which means "you are/do". Generally, 'wyt ti?' is given as the question form (like "dych chi?") and "rwyt ti" is the statement.
On the course, is it acceptable to say just "Ti" instead of "Rwy ti"? e.g. "Ti'n wych". Verbally and when writing informally or texting, I'll use ti. I know it might be a bit too lazy for a course though.