what word here means "Just", is acabaram de chegar an idiom that we should know?
Yes. Acabar de + verb = (have) just + verb. Ela acabou de sair = she (has) just left.
Why not " Dois ônibuses " ?
When you have a word that ends with an S (not oxytones), the plural is the same
O ônibus = os ônibus
O lápis = os lápis
O tênis = os tênis
but the "oxítonas' ( oxytones) have plural: inglês = ingleses; país = países; revés = reveses.
It rejected "two buses just arrived at the station". Yes, I know that this sentence is ambiguous and it could refer to a train station. But still, that's pretty picky.
in english English, we usually DON'T use this construction in this context, though Americans do. 'have just arrived' would be more usual, but is marked wrong.
"Depot" should probably also work. I reported it, but I also mention it here because no one else has.
Thanks, it is " with an s ".
What's the difference between "rodoviária" and "ponto de ônibus" please?
Rodoviária is a bus station like where you get a bus to go long distances or transfer from the subway to a bus. Ponto de ônibus is a bus stop like where you wait on the street for the bus to pick you up.