dylwn - ought to/should
There are two ways of saying 'could' in Welsh, using 'dylwn' (south) and 'dylswn' (north). Neither are wrong, but it's best to be aware of both, and let local usage dictate which one you use. As the northern variation is simply the same as the southern, but with an 's' following the 'l', only 'dylwn' is listed here. Do not use 'yn'.

"I ought to", "I should" 
"Ought I?", "Should I?"
"I ought not", "I shouldn't"

affirmative
dylwn i 
dylet ti 
dylai fe/fo
dylai hi
dylen ni
dylech chi
dylen nhw

negative
ddylwn i ddim
ddylet ti ddim
ddylai fe/fo ddim
ddylai hi ddim
ddylen ni ddim
ddylech chi ddim
ddylen nwh ddim

interrogative
ddylwn i?
ddylet ti?
ddylai fe/fo?
ddylai hi?
ddylen ni?
ddylech chi?
ddylen nhw?