Questions & Answers about Moja córka umie już czytać po polsku.
It is in the nominative singular.
That is because córka is the subject of the sentence — the person doing the action of being able to read.
You can think of the structure as:
- Moja córka = subject
- umie = knows how / can
- już = already
- czytać = to read
- po polsku = in Polish
So córka stays in its basic dictionary form here.