gallwn and medrwn - could
There are two ways of saying 'could' in Welsh, using 'gallwn' (south) and 'medrwn' (north). Neither are wrong, but it's best to be aware of both, and let local usage dictate which one you use. Do not use 'yn'.

"I could" 
"Could I?"
"I couldn't"


gallwn (south)

affirmative
gallwn i
gallet ti
gallai fe
gallai hi
gallen ni
gallech chi
gallen nhw


interrogative
allwn i?
allet ti?
allai fe?
allai hi?
allen ni?
allech chi?
allen nhw?


negative
allwn i ddim
allet ti ddim
allai feddim
allai hi ddim
allen ni ddim
allech chi ddim
allen nhw ddim


medrwn (north)

affirmative
medrwn i
medret ti
medrai fo
medrai hi
medren ni
medrech chi
medren nhw


interrogative
fedrwn i?
fedret ti?
fedrai fo?
fedrai hi?
fedren ni?
fedrech chi?
fedren nhw?


negative
fedrwn i ddim
fedret ti ddim
fedrai fo ddim
fedrai hi ddim
fedren ni ddim
fedrech chi ddim
fedren nhw ddim