Agreed, it would be helpful as a learner to be able to choose between North and South.
I think a better approach would be to have two units when differences like this crop up: so 'past tense south' and 'past tense north'
So with canu (sing):
mi wnes i ganu I sang
mi wnest ti ganu you sang (singular familiar)
mi wnaeth o/hi ganu he/she sang
mi wnaethon ni ganu we sang
mi wnaethoch chi ganu you sang (plural or polite or both!)
mi wnaethon nhw ganu they sang
canais i I sang
canaist ti you sang (singular familiar)
canodd e/hi he/she sang
canon ni we sang
canoch chi you sang (plural or polite or both!)
canon nhw they sang
Is there any possibility to split the learning tree into a northern dialect branch and a southern dialect branch?
Could both these forms "mi wnes i edrych" and "edrychais i" also be translated by "I was looking" or not?
But unlike English "I did look at the car", which is emphatic, "Mi wnes i edrych ar y car" is not particularly emphatic.
Compare "Wnes i ddim edrych ar y car" and "I did not look at the car" -- where in both cases "wnes/did" does not serve to emphasise but merely helps to form the negative sentence. In Welsh, "to do" can also help to form not just questions and negative sentences but also regular (unemphatic) positive sentences.