I have completed all the Levels to the end of level 3, but, as I struggle with bydda I have decided to “re-sit” all the levels but doing about 3 challenges per day.
One thing has come up which is confusing me; “ddylen i ddim” means “I shouldn’t”, but I noticed that some of the spoken answers give “Na ddylen i ddim”, so I looked on the vocab which says that “na” is used to negate the subsequent phrase and this is where the confusion comes in. I’m thinking that if you negate a negative sentence it becomes a positive, e.g. “Not I shouldn’t do” would actually mean “I should do”, rather like the “I didn’t do nothing” so often used in English.
Would someone kindly clear it up for me, please?
Diolch,
Tony
Unlike in English, it’s perfectly fine to use a double-negative in Welsh - it stays negative 
Thanks Siaron, does that mean it isn’t always necessary then, If I just use Ddylen i ddim it would mean the same thing?
(Edited)
Sorry that this is a bit after the event. As it happens, I had been checking this out for myself, so I hope that this will help.
Extract of your comment:
One thing has come up which is confusing me; “ddylen i ddim” means “I shouldn’t”, but I noticed that some of the spoken answers give “Na ddylen i ddim”, so I looked on the vocab which says that “na” is used to negate the subsequent phrase and this is where the confusion comes in.
Your 2nd example (the one including “na”) comes towards the end of the challenge, as a revision. We will need the “na” there because it means “that” before the negative clause: Blah blah blah, that I didn’t. In Welsh we can’t leave out the na/that in the way (that) we can in English.
PS in a positive/affirmative clause, we would use “bod” instead.
Also see another topic, here entitled “that they didnt”?("that they didnt"?)