galla and medra - can
There are two ways of saying 'can' in Welsh, using 'galla' (south) and 'medra' (north). Neither are wrong, but it's best to be aware of both, and let local usage dictate which one you use. The only time 'medru' wouldn't be used is in the sense of permission, when 'cael' would be used instead. Do not use 'yn'.

"I can" 
"Can I?"
"I can't"

galla (south)

affirmative
galla i
galli/gelli di
gall e
gall hi
gallwn ni
gallwch /gellwch chi
gallan nhw

interrogative
alla i?
alli/elli di?
all e?
all hi?
allwn ni?
allwch/ellwch chi?
allan nhw?

negative
alla i ddim
alli/elli di ddim
all e ddim
all hi ddim
allwn ni ddim
allwch/ellwch chi ddim
llaen nhw ddim


medra (north)

affirmative
medra i
medri di
medru fo
medru hi
medrwn ni
medrwch chi
medran nhw

interrogative
fedra i?
fedri di?
fedru fo?
fedru hi?
fedrwn ni?
fedrwch chi?
fedran nhw?

negative
fedra I ddim
fedri di ddim
fedru fo ddim
fedru hi ddim
fedrwn ni ddim
fedrwch chi ddim
fedran nhw ddim