It goes with the verb when it has a object. To "bjuda på X" is to treat someone to something.
Jag bjuder på nästa öl = Next beer is on me
Kan jag bjuda dig på något att äta? = Could I treat you to something to eat?
However, if standalone, på shouldnt be there.
Vill du ha mat? Jag bjuder! = You want food? I'll treat you to it!
There will be. There are quite a few verbs where the following preposition is important, and even where it might change meaning depending of whether you stress the verb or the preposition. To komma på is perhaps the worst example of this.
Jag kommer på det = I'm coming up with it, i.e inventing/finding out something.
Jag kommer på det = I come on it, as in the sexual kind of way.
Good explanation, Zmrzlina.
When I was originally taking classes in Swedish I remember being shocked to find that all of the verbs I had diligently learned changed completely once someone stuck a preposition after them.
But then I started noticing how often we do the same thing in English. It's really easy to come up with examples. We "punch in" at work, or "hop to it" when we're in a hurry. We "get over" a disappointment, like experiencing a "stick up"!
I think this is a tough call. "They are not paying for the food" can certainly mean "They are not treating", but could also mean "De betalar inte." Without more context the English sentence is ambiguous. I can see why the DL moderators wouldn't want to allow it ... although personally I'd like to see it accepted because I always get this sentence wrong. :-)
It's a tricky sentence because it is very ambiguous from Swedish to English. I agree with you and you should report it.
In Swedish it can basically describe four different situations: 1) They do not provide food (Would more likely be "ERbjuder" though) 2) They provide food but it is not free ("Bjuder de på allt det här?") 3) They provide other stuff but not food ("Bjuder de både på mat och dryck?") 4) They will not pay for [someone else's] food ("Ingår kostnaden för allt i resan?")
The main Swedish sentence is always accepted when translating back into Swedish. If you were shown a correction with erbjuder, you must have input something that the system thought was similar to that. Maybe you forgot the preposition for instance? The system will try to match whatever you input with the closest possible translation. If you don't input anything, it will only give you the 'best' solution, which in this case is De bjuder inte på mat.
bjuda på (mat) - offer (food). The Swedish verb is a phrasal verb in this case. English has other phrasal verbs also, where you have to learn what preposition to use for different meanings. For example 'put' has an large amount of different prepositions to change its meaning.