Breakdown of Czyje są te ślady na kafelkach, które dopiero co umyłem?
być
to be
ten
this (m)
na
on, at / to (motion toward) / for (a meal) / for (waiting for)
czyj
whose
Questions & Answers about Czyje są te ślady na kafelkach, które dopiero co umyłem?
Does dopiero co require a specific verb aspect or tense here?
The particle dopiero co means "just" or "just now" to show immediate anteriority. It doesn't use a special grammatical tense like the English present perfect. It simply pairs with a standard past tense verb, almost always perfective like umyłem (I washed), to emphasize a freshly completed action.
Does które refer back to the footprints or the tiles, and what case is it?
From context, you wash tiles, not footprints, so it refers back to kafelkach. Grammatically, które is in the accusative case here because it acts as the direct object of umyłem. It takes the non-masculine-personal plural form because the base noun kafelek is masculine inanimate.
What is the base form of kafelkach and why does it look like this?
The dictionary form is kafelek (tile). Notice that it drops the mobile "e" when declined. Because the footprints are located on the tiles, the preposition na triggers the locative case, resulting in the locative plural form kafelkach.
Why do we use czyje instead of kogo to say "whose"?