"The boy goes to the city with the girl."
Translation:Puer cum puella ad urbem it.
it is singular 'he/she/it goes'. We use it here since 'the boy goes'. Only puer is in the nominative, puella here is in the ablative (because of cum).
eunt is plural 'they go'. Would work if this sentence was something like puer et puella ad urbem eunt (the boy and the girl go to the city). Both puer and puella in this case are in the nominative.
"Puer cum puella it ad urbem." was marked wrong and I'm a little confused. "Puer cum puella ad urbem it." is considered the correct answer. I also tried out: "Puer it cum puella ad urbem." and that worked.
Is there a rule I'm not gleaning? Or did I just happen to pick a correct combination that just wasn't added yet?