[wodenokoto]

That's a great anecdote! I don't think I knew how to produce the ordinals above 39 consistently for most of my life either. And I've probably said "fyrrende" instead of "fyrretyvende" more than once!

That's the thing about irregular words: if they are rare, native speakers tend to use the regular form, and in this case we have a peculiar situation where there is no regular form!

When learning the 10's in Danish, I think it is very important to leave logic behind and not look for a system. As a kid I had a lot of trouble with 50 and 60 and eventually settled on "the one with "half" is smaller than the one without" and did the same with 70 and 80.