I am not clear on when to use medrwn i or o’ni’n medru. It is my understanding that they can both mean 'I could". Any help would be greatly appreciated.
The verb medru is “to be able”. The conditional form (I could) is medrwn i (e.g. If it’s dry today I could go for a walk = tasai hi’n sych heddiw, medrwn i fynd am dro)
Technically, o’n i’n medru is saying “I was able”, but of course we also tend to translate this as ‘could’ though it’s a different kind of ‘could’ - i.e. not a conditional (e.g. I was able to / I could… swim faster years ago = o’n in medru nofio’n gyflymach blynyddoedd yn ôl).
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What’s the difference between medru and gallu, please @siaronjames?
I was thinking that one was northern and one southern, but I’ve recently come across a northern speaker using them both (and even in the same sentence!)
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It’s pretty subtle. On a basic level (“to be able to”), there’s no real difference, but when you get into the real subtleties…
These definitions are from GPC:
Gallu : to be able (to), have power (to), can, be able to accomplish (a thing) .
Medru : to be able to do or accomplish (a thing); be able to speak, know (a language); be proficient or well-versed in (e.g. an art, work), be conversant with, know (of), comprehend, possess (skill, &c.); know by heart .
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Oo, thanks Siaron! That is subtle!! 
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