"The trains are not new."

Translation: A vonatok nem újak.

[brjaga]

When do you use -ok, and when -ak? I would have expected aCa and uCo, but instead it seems to be the opposite

[MrtonPolgr]

You can't distinguish -ok and -ak just by looking at the last vowel - both can appear with a back vowel word and basically you just have to learn it. These two words can be considered regular as the majority of nouns take o whereas the overwhelming majority of adjectives take a.

[kyavila]

What is the difference between vonatok and vonatokat?
I thought it was vonatok - plural of trains
vonatokat - accusative plural of trains. Please, someone explain ^v^

[vvsey]

Yes, that is correct. What is it that you don't understand?

[kyavila]

I wrote "A vonatokat nem ujak" because I thought it would be accusative but it was marked wrong.

[vvsey]

I see. "Vonatok" is the subject of the sentence, it is in the nominative. We use the accusative when the noun is the direct object of a verb.

"Én nem látok vonatokat." - I do not see trains.

"Én" is the subject, it is in the nominative. "Vonatokat" is the object, it is in the accusative.