Special declensions: dio, radio, taksi

dio hides an l

dio (part) looks indeclinable and is anything but. Like posao → posla, its final -o was once an l — but dio also changes its vowel: dio → dijela, dijelu, plural dijelovi. Pročitao sam samo dio knjige, a drugog dijela se ne sjećam.

Foreign nouns keep a -j-

Nouns borrowed with final -i or -io insert -j- before Croatian endings: taksi → taksija, radio → radija, studio → studija. Čekamo taksi ispred hotela; bez taksija ne idemo. Slušali smo vijesti na radiju.

The locative u studiju (in the studio) and genitive tijekom studija can also come from studij (university study) — context decides, and both decline with -j-.