> 1. The ending -r > -ur after consonants, e.g. maðr > maður, Hildr >
> Hildur. Also where -ur is part of the root, e.g. fagr > fagur,
> fagrt > fagurt. This doesn't apply where a vowel follows in the
> ending (nökkrir > nokkrir), except with the suffixed definite
> article: maðrinn > maðurinn.

It's even more tricky than that.

hestur > hesturinn
vetur > veturinn
(Hildur > Hildurin)

but

mastur > mastrið

Kveðja,
Haukur