"아니요, 여자아이는 남자가 아닙니다."
Translation:No, girls are not men.
Context/it's not necessarily as important as we (people who grew up with western/European languages) think it is. If you know whom the speaker is talking about, then you already know whether it's singular or plural. If the speaker is making a general statement (this sure sounds like one), then you can translate either with plurals ("Girls are not men") or with the indefinite article "a" ("A girl is not a man") because both have the same meaning to express a general statement. If you know it's not a general statement, but don't know whom or what the speaker is talking about, then you generally don't know whether to translate it singular or plural--but in that case, since you don't really know what they're talking about, singular or plural won't really help much.
It is the "subject marker." A subject marker (가 after a vowel or 이 after a consonant) indicates who or what is performing the action in a sentence. Contrast this with the topic marker (는 after a vowel or 은 after a consonant), which indicates the object of the sentence or who/what you are talking about.
This sentence of Hangul should be more clearly expressed in plural and singular form.
' 아니요, 소녀( girl )는 남자( man )가 아닙니다. '
' 아니요, 소녀들( girls )은 남자( men )가 아닙니다. '
But in Duolingo's English class, it's a little bit more differentiated. So what I want to say is that they should ask Korean more clearly.
Hi! I'm confused by the 남자가 and 여자아이는, as I was under the impression that 가 is for subjects (performing the action, here Girls, who ARE not men, performing the verb TO BE) whereas 는 is for topics being talked about. Even then I could understand that the girls are being talked about here so we have a 는 rather than 가 (though it's confusing when to use which if both are applicable sometimes) but why does 남자 have 가 after it here? The men are not performing an action and are not a main subject, the girls are. How does this work? Why is it not 여자아이가 남자는 아닙니다 ?
Topic markers 은/는 are used for specific things. For example: 친구는 자동차는 있습니다 = The friend has THE car. This is talking about a specific car.
Subject markers 가/이 are for general things. For example: 친구가 자동차가 있습니다 = A friend has A car. This isn't being specific, its just talking about, in general a friend has a car of some sort.
Think of it like in school. You have subjects like maths or english but in those subjects you have specific topics you learn, like algebra or poems.