Does anyone have a clue why is that word changing so radically from nominative (miasto) to locative (mieście)? :) There seems to be some irregular things in every language that don't have deeper reason, is this one of them? Dziękuję bardzo!
Two regular processes:
normal palatalization: hard -st- → soft -ści-
-a- between two soft consonants usually turns into -e-
It's regular alternation of a stem. "Miasto" belongs to III group of declension of neuter nouns. Look at "miejscownik" (locative):
Alternation occurs according to this table: bit.ly/21H2hPE