Breakdown of A data em que ocorreu o batismo dele é desconhecida.
ser
to be
em
in / on / at
dele
his (forms: dela, deles, delas)
a
to / at (direction)
Questions & Answers about A data em que ocorreu o batismo dele é desconhecida.
Why is the verb ocorreu placed before the subject o batismo dele?
This is an example of subject-verb inversion in a relative clause. When a relative clause (starting here with em que) has a longer or heavier subject like o batismo dele, the verb often comes first to make the sentence flow better. While you could say em que o batismo dele ocorreu, the inverted form em que ocorreu o batismo dele sounds much more natural in Portuguese.
Why do we use em que here instead of just que or onde?
In Portuguese, events happen "in" a date (numa data), so the relative pronoun connecting back to A data must carry the preposition em. We cannot use onde (where) because onde is strictly reserved for physical locations, not points in time.
Could I use o seu batismo instead of o batismo dele?
Why does the sentence end with desconhecida instead of desconhecido, given that batismo is a masculine word?