byddwn and baswn - would
There are two ways of saying 'would' in Welsh, using 'byddwn' (south) and 'baswn' (north). Neither are wrong, but it's best to be aware of both, and let local usage dictate which one you use. Use 'yn' and 'wedi' with byddwn.

"I would" 
"Would I?"
"I wouldn't"

byddwn (south)

affirmative
byddwn i
byddet ti
byddai fe
byddai hi
bydden ni
byddech chi
bydden nhw


interrogative
fyddwn i?
fyddet ti?
fyddai fe?
fyddai hi?
fydden ni?
fyddech chi?
fydden nhw?


negative
fyddwn i ddim
fyddet ti ddim
fyddai fe ddim
fyddai hi ddim 
fydden ni ddim
fyddech chi ddim
fydden nhw ddim


baswn (north)

affirmative
(ba)swn i
(ba)set ti
(ba)sai fo
(ba)sai hi
(ba)sen ni
(ba)sech chi
(ba)sen nhw


interogative
(fa)swn i?
(fa)set ti?
(fa)sai fo?
(fa)sai hi?
(fa)sen ni?
(fa)sech chi?
(fa)sen nhw?


negative
(fa)swn i ddim
(fa)set ti ddim
(fa)sai fo ddim
(fa)sai hi ddim
(fa)sen ni ddim
(fa)sech chi ddim
(fa)sen nhw ddim

Note: sometimes the 'ba' of 'baswn' is dropped, resulting in 'swn i', 'set ti', 'sai fo' etc.