"They see the flag."
Translation:Ili vidas la flagon.
I see! It totally makes sense.
I was confused comparing "Ili vidas la flagon" with "Ili estas bona" (not "bonan"), for example.
With a little bit of research, I understood that "vidas" acts as a transitive verb, that's why "flag", as a direct object, gets an "-n", while "bona" is not an object to "estas", that's why it doesn't get the "-n".
Am I right in this assumption?
In the right context, I would say this would be a good translation. My hunch, though, with no extra context, is that this clearly refers to a specific "they" seeing the flag.
John, Mary, and Mark are in America - they see the flag.
They drive on the right side in America - this would be "oni".