I came across this …do’n methu gadael…which to me says… I wasn’t unable to leave…but the person who saying this was meaning… I wasn’t able to leave.Should the correct way be…roeddwn i’n methu gadael or o’n methu gadael…as do’n means I wasn’t.
In the South we often use “ffili” instead of “methu”.
I share your view of the apparent double negative, but I’ll wait to hear from someone who knows 
Double negatives don’t affect Welsh the way they do English. So you can say “do’n i’n gwneud dim byd” and it doesn’t sound crass like “I wasn’t doing nothing”. (You can also use unrhywbeth)
So “i was unable to do something” is a negative sentence, therefore, Welsh makes the whole thing negative. “Do’n i methu gadael…”
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I’ve also thought of another way of explaining it:
I was not able
I was unable
Both negative sentences.
Do’n i ddim yn medru
Do’n i’n methu
Both negative sentences.
Even though, I’ve also found:
rhywbeth chi methu dianc rhag
o’n i methu pasio
o’n i methu canolbwyntio
I guess you can use both…positive+negative and negative+negative?
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You can yes. Just with “o’n i methu aros at…” (I couldn’t wait) as a declaration of excitement (positive sentence). It’s a useful friend methu.
Main point is, double negatives aren’t an issue in Welsh
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I hope this isn’t going to add a layer of confusion, but if you really want a double negative as in for example “I couldn’t not laugh” you can use peidio - “O’n i methu peidio chwerthin”.
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I learned early on that double negatives are not only acceptable but grammatically correct in Italian as in “non ho niente / I don’t have anything” or (more appropriately for me) “non so niente / I don’t know anything”
I wonder if there is an analogue in Italian of @robbruce 's use of peidio to translate an exchange like this
“You know nothing”
“I don’t know nothing, I know a lot.”
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The analogue in Italian of @robbruce’s example is very easy.
Just add one more “non”, and they will annihilate one another! 
Non potevo non ridere.
However, trying to figure how to translate the exchange you mention…maybe you need to change it around a bit like this:
“You know nothing” - Tu non sai qualcosa.
“I don’t know nothing, I know a lot.” - Io non so qualcosa, so molto.
And in Welsh, how would that be
Ti’n gwbod dim byd
Sai’n gwbod dim byd, dw i’n gwbod llawer
? 
I’m confused now!
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Diolch tanto
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