Learn Polish: Mój ojciec pracuje w biurze. - My father works in an office.

Because it follows the preposition w (in), which requires the Locative case to describe a location. For most neuter nouns like biuro (office), the -o changes to -e. Notice that the last consonant also changes: the r becomes rz to make pronunciation easier, resulting in biurze.

Could I use na instead of w for "in an office"?

No, you need to use w. In Polish, w is used for enclosed physical spaces, rooms, and buildings like an office (w biurze). The preposition na is used for open surfaces, events, or certain specific institutions like the post office, but a standard office takes w.

The dictionary form for "to work" is pracować. Where does the "uj" in pracuje come from?

This is a common pattern you might remember from earlier lessons: verbs ending in -ować drop the -ow- and replace it with -uj- in the present tense. Since ojciec (father) is third-person singular (he), we add the -e ending to get pracuje.

Why is it mój and not moja or moje?