This is confusing for those learning basic Polish. When the verb conjugation is not unique to the subject, the subject shouldn't be omitted. It's natural for a Polish-speaker but not for a learner, to omit the pronoun/subject.
This would be like, translate from English: Often comes back to this book. He? She? It?
If the conjugation is unique, however, then it's clear. For example Często wracam do tej książki, it's understood that "I return to the book..."
Difference between Polish and English sentence is that English one is not grammatically correct, and Polish is 100% correct. Of course it would be better to provide context,
I understand that it's grammatically correct in Polish, and that's not the problem. The same problem in both languages is that you don't know who is being talked about. My opinion is that first- and second-person perspective, singular and plural, should be omitted, to avoid sounding redundant, for the purposes of learning Polish correctly. Third-person perspective, singular and plural, however, does not sound redundant or unnatural with the pronoun/subject intact, so they shouldn't be omitted. As a learner I believe this makes Polish much easier to comprehend and properly translate.
In this declension table: https://pl.wiktionary.org/wiki/Aneks:J%C4%99zyk_polski_-_zaimki#Zaimki_rzeczowne
It is written,
W języku polskim nie używa się w funkcji podmiotu zaimków 1. i 2. osoby, chyba że chcemy wyraźnie podkreślić, że to te osoby wykonują wyrażoną w zdaniu czynność. W innym wypadku zaimek pomijamy, gdyż to samo znaczenie wyraża końcówka czasownika (orzeczenia zdania). Przykład: Napisałem książkę. zamiast Ja napisałem książkę.
It doesn't say anything about omitting the 3rd-person pronouns.
The difference is that you should omit first and second person pronouns, unless you really want to point out who is doing something. You can omit third person pronouns when it's clear from context. And certainly you can omit pronouns in the past tense, when you can tell gender from the verb form.
Unfortunately there is never context on duolingo.
That's why third-person pronouns should not be omitted. There is no context in Duolingo and it's confusing in translations to have 3 correct answers he, she, it.
Yes, and without context, ALL these options are possible. We do omit 3rd person pronouns as well, even if not that often - whether you like it or not. It's better to get the users confused right now, in an online language course, sitting in front of their computers/phones; then for them to be confused later in real-life situations.
You never had and never will have any context in Duolingo sentences, which has its pluses and minuses. And even if there's no context, this is a perfectly correct Polish sentence - it just is. It's better to get accustomed to it. Some sentences will always be ambigous. Just like in English every sentence using "you" will be ambigous and it will cause the same kind of problem - is it singular, or is it plural? And then you just have to choose.
Yeah of course! Thank you! But would it not be relevant to define who comes back, or does it not matter in Polish?
I made a mistake, and translated it to "This book often comes back"
It should be clear from context about who you speak. You could insert name or proper pronoun but it can be obvious from context.
I wrote she often comes back to this book and was marked wrong. It was he. How do I tell it was he not she