When dealing with money in Welsh, we use the singular punt and ceiniog most of the time- un bunt, tair ceiniog etc.
When you get up to mil, it switches from singular to plural, along with adding on the extra o- ugain mil o bunnoed, pum mil o bunnoed etc.
Is there a particular reason for this difference? I know most numbers up to 100 are considered masculine (except 2,3,4), but why are mil and miliwn feminine nouns, and where does the o come from?
There’s no reason for this difference, and having the switch occur at a thousand is just an arbitrary point. You always have the option when using number words to use the singular, or the plural with an o inserted:
Mae gyda fi dri phlentyn. and
Mae gyda fi dri o blant. are interchangeable.
In the days when Welsh was still Britonic Celtic, and had case-endings like Latin, they used to say “three of children”. But (insert something about Latin and ancient Gaulish, yada yada yada) the ending for “of children” looked exactly like the accusative singular “child”, and so Welsh wound up with its slightly odd “one child, two child, three child” system. Fast-forward a millennium or so, and the o is just Welsh going back to its “three of children” roots…
The switch is generally heard in larger numbers well before mil, but as Hendrik said, there is no hard rule as to when and both are acceptable with all numbers - it just sounds more natural to use o with some numbers and not with others, so it’s more a feeling for what sounds right rather than a rule.
because there are always exceptions to any rule! 