I find the partial sentences like “that we know how to do it” very off-putting. Why can’t we have proper sentances with a main verb like normal speech? Translating 'that" needs a proper context. Real speech hardly ever begins with “that”.
One of the ways the course works is to give you blocks of speech patterns that you can integrate into patterns you have already learnt rather than being a kind of ‘phrase book’ of entirely ready-made sentences. It makes your brain work a bit harder, but brings greater benefits in doing so.
This may work for some phrases but I do not think it does for subordinate clauses starting “that”. When I hear “that” have no idea whether to say bod, fod nad or whatever.
On another point is i really accptable to have both “nad” and “dim” in the same clause, as sometimes happens in Level 3? My grammar book is clear that they are alternative forms of negation.
@siaronjames will know better than me, but I believe that some people in north Wales do indeed add this ‘redundant’ ddim into those phrases, e.g. dw i’n gwybod nad ydy hi ddim yn dod heno. It sounds a bit weird to me, but I know it appears in the northern version of the course fairly frequently.
At least in spoken Welsh, double negations are usually acceptable. And as SSiW teaches natural spoken Welsh (rather than formal or literary Welsh), you may come across bits and pieces that are contrary to traditional grammar and literary standards.
As an aside, the leading D in negative forms is a remnant of the negative particle ni(d), so in a sentence as Dydw i ddim yn yfed te you kind of have a double negation anyway.
Sorry - late back to the party - been incredibly busy working this weekend. As Hendrik said, it’s very common in natural spoken Welsh to come across things that don’t fall in line with grammar books or dictionaries. If you’d rather stick to fully grammatically correct forms, there’s no problem in doing that, but the natural colloquial forms are just as valid in speech.