은 indicates regardless of other girs, this girl specifically. Hence, "the." 여자이가 is more indiscriminate and closer to "a girl."
While it does that, too, it indicates a general statement as well, where in singular you would use "a" in English.
Yes, because the answer you were suggesting is a plural fragment. It says that the girl is reading ONE difficult book, yet it kind of stumped me too. Oof.
But "들" is supposed to be optional when the context is understood, and there are times when Duo will require a plural without it, so either should be accepted. In English, it would be more natural to use the gerund for a single book - The girl is reading a difficult book. Duo won't allow that translation.
why is hard, as in a hard book, wrong? difficult and hard were both offered as meanings...
Small is a descriptive word, so a girl that is small (in size, height ect) as opposed to a large girl. Small does not mean just young when referring to a person