Learn Croatian: Naš sastanak počinje u pola deset. - Our meeting starts at half past nine.

In Croatian, when you use pola (half) for time, you state the upcoming hour, not the current one. It literally translates to "halfway to ten". So, pola deset is 9:30, pola osam is 7:30, and so on. Be careful, as this can be very confusing for English speakers!

Why do we use the preposition u here? Doesn't u usually mean "in"?

Yes, u commonly means "in" or "into" when talking about physical locations. However, when telling the time on a clock, u is always the preposition used to mean "at". So "at nine o'clock" is u devet, and "at half past nine" is u pola deset.

Why is it Naš sastanak and not Naša sastanak?

The word sastanak (meeting) is a masculine noun, which you can usually guess because it ends in a consonant. Because it is masculine, the possessive pronoun "our" must also take the masculine form, which is naš.

The verb here is počinje. I've also learned početi for "to start". What is the difference?

počinje comes from the verb počinjati, which is the imperfective partner of the perfective verb početi. In Croatian, present-tense scheduled events (like a meeting starting) use the imperfective form. You would use početi to talk about a single, completed action, usually in the past or future.