'Why won’t “The boy with the girl go(es) to the city” work?'
Yes, in English that would work.
You can say “The boy with the girl goes to the city”, or better yet, “The boy, with the girl, goes to the city”.
Duolingo's translations aren't perfect, and I'm guessing, as a native English speaker, that Duolingo's English translations have a lot of problems, because English has many exceptions.
'I knew what they meant'
First comment, but I am coming across a few ... conceptual obstacles. Coming from IT, where we say 'abstract' things like "foo" and "bar" to help each other along, I realize it may just be my frame of reference and my being overly reliant on conventions of my native language, (American) English.
In this case, I knew the 'meaning', but had to decide if they would like a 'direct translation' or a 'meaningful translation'. I could be wrong, but there seems to be a little bit of flip-flopping from the expectations there. Sometimes, the correct answer is the one where you 'reorganize' the appearance of words to make the sentence 'sound good' in the 'other' language, which ever it may be at the time. Other times, it seems to want the obtuse, borderline/pseudo-shakespearean version of things such as, 'the boy with the girl to the city goes'.
It says 'the boy with the girl to the city goes', but I know it 'sounds better' when it is reorganized to 'the boy goes with the girl to the city'.
There was another phrase, same deal... EXCEPT the obtuse version would be totally acceptable - to someone who is actively trying to demonstrate their dominance by being flagrantly passive aggressive. So, in that (those) case(s)... we have to stop even reading it as dialogue.
I am having a great time nonetheless, and I think perhaps there is something I have yet to learn as a student of Latin which will help me navigate this going forward. I also can't propose an 'easy fix' ... other than maybe giving all the sentences another once-over for absurdity and/or 'questionable tone issues' that may interfere with the learning at hand.
No, sorry but, do you recognize the difference b/w these sentences? "The boy with the girl (there are 2 boys, one with a girl, the othe with a cat. Here talking about the one with the girl) goes to the city (and the boy with the cat stays"; and "The boy goes with the girl to the city (this may be better to say The boy goes to the city with the girl)