"Who is going on the bridge?"
Translation:Kik mennek a hídon?
The Hungarian sentence is talking about the action of people who are already on the bridge, not the action of getting onto it. From the English sentence there's no real way to tell whether -ra or -on is the intended meaning - "going on" could mean either "in motion on" or "moving onto", although I would say that the onto, -ra meaning would more usually be taken.
Good question. The difference is like "you are walking ON the main street" and "you are walking TO (towards) the main street". "Kik mennek a hídra?" would be "Who is going to / towards the bridge", so "-ra/-re" used with a destination. "Kik mennek a hídon?" means "Who is walking on the bridge", so "-on/-en/-ön" used with a surface in which the current action is performed.
To add to this, I think it is the choice of verb that makes that small difference in English.
Going vs walking.
With an extreme example:
- I am going on the Moon - I am going TO
- I am walking on the Moon - I am already there (can you believe it?), I am walking on the surface
That is why "going" and "walking" are used differently in English and in Hungarian. "Going" is not really used to describe a general motion, without a source or a target reference. It is a directional verb. "Walking", however, can be used that way, just to describe a moving action:
- I am walking on the street
- I am going on the street - ambiguous, at the least
Hungarian, on the other hand, makes a clear distinction between "on (the surface)" and "onto (a target)".
And that is why Hungarian can use the same general verb, "menni", to describe both situations. Because the suffix makes it perfectly clear:
- Megyek a Holdra - I am going to the Moon
- Megyek a Holdon - I am on the Moon and I am in motion
And this is also why the verb "to walk" is frequently better translated as "menni", not "sétálni".