Prepositions, again

I'm back at this for the holidays. I think prepositions ending with a dental stop are going to change their forms.

kot will become ko, from
nad will become na, above, over
niúrut will become niúru, in the middle
kaúrut will become kaúru, to the middle
ishiúrut will become ishiúru, from the middle

These forms will be used before consonants. Before vowels they will end in a d.