"Do you have a boyfriend?"
Yeah, they went informal out of nowhere. It's important to learn, and I get why they wait so long to start teaching informal speech, but it can be very confusing. I actually got this sentence for the first time in the very last lesson in level 5, which really threw me.
Duolingo is great, but if it's your only resource, you're gonna get confused a lot.
恋人はいるの？ こいびと は いる の？
恋 = こい = Love 人 = ひと = Person 恋人 = こいびと = Lover/bf/gf https://jisho.org/search/%E6%81%8B%E4%BA%BA
いる is the informal form of います. So you could say 恋人はいますか？
の is similar to か in that it poses a question. https://jisho.org/search/%E3%81%AE On jisho, it is the 6th usage. "6. (at sentence-end, rising tone) indicates question"
Also idk if duolingo accepts it, but yes 彼氏はいますか？ should be fine. This is all to the best of my knowledge. Correct me if I am wrong.
If new words are introduced into a lesson, please introduce them in the first levels with a hearing exercise, a matching exercise, a single recognition exercise AND a phrase translation exercise. Don't just throw a translation exercise in the very last level of this lesson, that's pointless and not helpful