Learn Dutch: Zullen ze de koffie serveren als de garderobe is geopend? - Will they serve the coffee when the cloakroom is open?

This sentence uses the state passive. We use is (a form of zijn) plus the past participle geopend because we are talking about the resulting state of the cloakroom (the fact that it is open). If we used wordt geopend, it would describe the action itself, meaning the coffee would be served at the exact moment someone is physically opening the doors.

Why are the verbs at the very end of the sentence (is geopend)?

The second part of the sentence starts with als (when). This word introduces a dependent clause, which is a grammar rule you likely remember from earlier lessons: it forces all the verbs in that part of the sentence to the very end.

Could I say geopend is instead of is geopend?

Yes! At the end of a dependent clause, you can put the helping verb (is) before or after the past participle (geopend). Both als de garderobe is geopend and als de garderobe geopend is are perfectly correct and sound natural.

Could we just use the adjective open instead of the past participle geopend?

Yes, you could also say als de garderobe open is. Using geopend (opened) sounds slightly more formal or official, but in this context, they both describe the exact same resulting state.