"The healthy boy with the girl counts olives."
Translation:Puer sanus cum puella olivas numerat.
What an odd order of words to use in English -- unless you are wanting to make some kind of differentiation, e.g. "The healthy boy (who is) with the girl" as opposed to, say, "The healthy boy (who is) with the sick parrot". But the Latin sentence isn't trying to do that (in fact, it can't, using these words alone).
Not necessarily. The boy could go to class, there could be 30 other students with him, but if I ask you what the boy is doing, you'd still say "he is studying with the other students," rather than "he are." Your utterance is singular because it only concerns itself with one out of the 31 people who are doing the action; the others are irrelevant, linguistically, in this context.