I don't understand, does the verb conjugate depending on the definiteness of the object, or was the point of the lesson to recognise that there needs to be an article, so only the definite form was correct?
What do you mean? You can not say "Băiatul mănancă măr" (as in an apple, where it becomes "un măr", never just "măr"), because the final "ul" is equivalent to "the"
the conjugation of the verb has nothing to do with the definiteness or undefiniteness of the object ....
You use the same verb form ( here 3 rd pers. sing. "mănâncă ") concording to the SUBJECT "Băiatul" to express that
"the boy eats an apple" or that "the boy eats the apple"....
the clue in this exercise is to recognise that the only thing he can EAT is an object like : an apple ( "un măr" ) or the apple ( "mărul" )
the five other listed objects were things you can DRINK ....
I have looked through the notes and tips and still don't uderstand- when and why does it become mărul as opposed to un măr? Thanks!