Questions & Answers about Koncert začíná teď.
It is nominative singular, because it is the subject of the sentence — the thing that is starting.
Začíná is the 3rd person singular present tense form of začínat.
It matches the subject koncert, which is singular:
- já začínám = I start / am starting
- ty začínáš = you start / are starting
- on/ona/ono začíná = he/she/it starts / is starting
Since koncert is treated like it, Czech uses začíná.