"He is a man and I am a boy."
Translation:On jest mężczyzną, a ja jestem chłopcem.
But can't one infer the subject from the conjugation, i.e. jestem means "me"?
Yes, on its own "Jestem chłopcem" would be not only perfectly fine, but a lot more natural than "Ja jestem chłopcem".
But here, as immery said, the subject changes in the middle of the sentence, you do have the emphasis on the pronoun, and therefore it is necessary.
First thing is that one should use „a” here. The thing we can omit here is „jestem”:
„On jest mężczyzną a ja chłopcem”.
Here we cannot skip „ja”. You can attribute this fact to a change of subject.
You can omit "to be" in the second part, but you have to leave it in the first one, otherwise we don't know what the verb is.
One: typo in "chłopcem".
Two: If X and Y are different things, there's a contrast between them - you have to use "a". I've seen someone comparing it to "whereas". So this is like "He is a man, whereas I am a boy".
The only thing wrong with my sentence was that there was no comma. Duolingo marked it as wrong. But the other time I used "i" instead of "a", also without the comma, and it marked it as correct. That's why I ask.
That should be accepted, maybe that's just a bug that it wasn't this time.
Actually, "i" was accepted but I deleted it, it doesn't suit the sentence. There are some situations when both work (and have a bit different meanings), but I don't think that it works here. After all, if X is a man and Y is a boy, that surely is contrast.