Neidr (Breton naer, Irish nathair), like English adder (which was rebracketed to an adder from a previous a nadder) and Latin natrix ("water snake"), come from Proto-Indo-European *nh₁trih₂.
I have a question about plurals. Are they optional in Welsh or are you just not teaching us yet because they;re quite irregular. Like is Naw neidr correct or should it be Naw nadroedd? Same with afal, is dau afal right or should it be dau afalau? Deg ci or deg cŵn?
You never use plurals directly after numbers in Welsh. So only "Naw neidr" never "Naw nadroedd" (same for the others.) Everything taught in the course is correct, we would never teach something incorret for the sake of making it simpler though we do sometimes chose to teach the simple/more common form.
The instruction about singular forms of nouns following a number is certainly in the Tips. But I think the question is more about why plurals have not been formally introduced. In Italian they were introduced at the 5th circle. As far as I can see they don't get a circle in Welsh, ever. Some explanation of why they're not being addressed would be useful.
There are perhaps 40 different ways of forming plurals in Welsh, so it is not feasible to teach them as a unit.
Some plurals are introduced in this course as and when they are useful in particular patterns. The best way to learn them is on a gradual basis.