A rule

I was wondering if anyone could formulate a rule for something I continue to get snagged on. Wnesi (ti) as a statement or question and the use of Oes

The only difference between a statement and a question is in fact the intonation; when asking a question the tone rises at the end, in a statement the tone is neutral.
Wnest ti wylio’r ffilm’na? - Did you see that film?
Ond wnest ti addo. - But you promised.

Oes is a special form of bod/to be, and it is used in questions asking for the existence of something (and in the echoing answer)
Oes plant 'da ti? - Do you have children?
Oes, mae dwy ferch 'da fi. - Yes, I have two daughters.