In Lesson 8 of Course 3 about 26:54, why is the sentence, “She won’t go if you don’t hurry.” translated as:
“Aeth hi ddim os na nei di frysio.”
rather than as:
“Aeth hi ddim os na nei di ddim brysio.”
Compare with the translation in a slightly later sentence:
“You’re lucky you didn’t come here first.”
“Ti’n ffoddus na ddes ti ddim yma yn gynta”.
Yours, confusedly,
Gerald
The “na” negates the sentence already so the following “ddim” is not needed but is used often in speech anyway.
Yeah, Tyger’s right on the money here - you’re well into ‘just because’ stuff here - there’s probably a case to be made about the feel of the sentence, and when adding ‘dim’ feels as though it fits with the rhythm, and when it doesn’t - but the key point is that you’re not meant to be understanding and remembering a rule here, just using whatever comes to mind first, and recognising whatever other people use 
Thanks for your prompt, helpful replies.
It sounds as if" …na ddudes i ddim" would be equivalent to an English construction “…that I didn’t say nothing”.
Would it be totally incorrect, however, to say “bo ddudes i ddim”?
You could say something like ‘ti’n ffodus bod ti ddim wedi…’ - but adding bod to a short form is (off the top of my head) always going to lead you to needing to use the ‘na’…
But this really is the kind of very close detail that will come much more naturally from getting into and listening to enough conversations more than from focusing on consciously controlling a list of rules, I promise
Thanks, Aran. I’m probably a hopeless case. I know that you advise simply listening; learning by building upon the translations you provide; and by conversations with Welsh speakers. However, sixty years ago when I was doing French and Latin at school we had to decline nouns in various cases and to conjugate verbs in tenses and moods so I still find myself trying to build rule sets for things like "when do you use “na” rather than “bod”; remember when using “na” to mutate what follows: or for other things like, is “iddo fo” a Welsh dative case or an accusative one?
Mea, culpa, mea culpa, mea maxima culpa,.
Still, I can’t believe how much Welsh I’ve learned in just over a year. Thanks to you and the team for that.
Gerald
It sounds as though you’re anything but! And you’re more than welcome to carry on asking about possible rules - if there are any handy, we’ll be happy to share, while we carry on preaching relentlessly about focusing on working through the lessons…
I remember that when I was learning French in the sixties, Gerald. Five years of French and none of us could speak a word of it…Speaking with young people even now it seems not a lot’s changed over the years.
Meanwhile, in another part of the forest, some of us have recently been discussing Caesar’s “De Bello Gallico”. Sounds like you’d be well at home there. 
Pax tecum.
I know this is old news, but I’m doing this lesson at the moment. I was a little freaked out not to be using ‘bod’. I do understand the use of ‘na’ in such sentences as: ‘Dw i wedi clywed na doi di ddim yn ol’ - I heard that you won’t come back
I also get that ‘dim’ is added because it kind of ‘feels right’.
So, my question is are there double negatives in Welsh, are they formed in a different way? If you use ‘na’ and ‘dim’ does it make it positive? Like in English you can say ‘I did not not say that’ to mean ‘I did say that’ albeit with a very different nuance.
Also does it sometimes create confusion with ‘yna’ to mean ‘there’ which is often shortened to 'na. So the example sentence could mean ‘I heard there [place] that you won’t come back’ as isn’t this sentence still ‘won’t’ as the ‘dim’ is there?
Might do sometimes for learners, but it’s the kind of thing that you’d very rarely be thrown by in your first language.
In terms of double negatives - you could deliberately build one, like ‘dwi ddim yn dweud bo fi ddim yn licio fo’ that would have the cancelling effect you’re talking about in English - but in normal terms, na/dim matches will still just be plain old negative…