"She often comes back to this book."
Translation:Często wraca do tej książki.
She vanished. In Polish you very often have an implied subject, known either from the form of the verb, or from the context. Here, the verb "wraca" only shows that it's 3rd person singular, so both "He" and "She" are equally correct when translating this sentence into English.
But if you had context, there would be no need to repeat the subject. If you were just talking about Anna and some book she likes, neither "Anna" nor "ona" would really feel natural in this sentence, as the context would make it obvious who comes back to this book.
Nothing. Both "he" and "she" are equally correct here... although this is an EN->PL discussion, I'd expect this question rather in the PL->EN one.
Most Duolingo sentences use 3rd person pronouns explicitly, but in real-life language, you will mostly have sentences like this one, because of the context. If you're talking about Adam, then "Adam often comes back to this book". If you're talking about Jane, then "Jane often comes back to this book". Such a sentence as this one assumes the 3rd person singular subject is already known.
But, Jellei, if you wanted to use the pronoun "ona", where would it be placed?
At the beginning. I think it's rarely natural to put the subject in a different place, that would change the emphasis. Possibly you could have "Często ona wraca do tej książki", but that's like "Often it is she who comes back to this book".
Często ona wraca do tej ksiażki (Polish a not possible with my keyboard) is marked wrong. Is it a word order problem? Or?
I get the business of the correct place for the subject, but surely with no context you do need 'ona' to properly translate the clause.
You know what's great, that duolingo teaches the word for "to come back", still I don't even know how to say simply the word in polish for "to come".