Wouldn't "Janos does not see" be a more accurate translation ? With something like "János nem látni tud" for "Janos cannot see". The meaning being "he can see -he is not blind-, but he isn't seeing sth right now"
Both "János does not see" and "János cannot see" are translated the same way into Hungarian. "Nem tud látni" is very rarely used.
Also, "János nem látni tud" would rather translate into something like "It is not seeing, what János can do". Word order matters a lot :)
I was marked wrong for "Janos is not seeing." Is that an error, or is there some difference in meaning I can't figure out?
"see" is a stative verb in its basic meaning and doesn't usually take the -ing form in English.
We say, "I know that, I love you, I see three ships, I don't think so, I hope not" etc. rather than "I am knowing that, I am loving you, I am seeing three ships, I am not thinking so, I am hoping not".
There is an alternate meaning of "see" (meaning "to go on dates with regularly") which does form the -ing form.
So "János sees Pál" means that János opens his eyes and notices Pál with them, but "János is seeing Pál" means that the two are going out together frequently. They don't mean the same thing.