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..."
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.
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.
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.
There's a lot of discussion about omitting the pronoun here. I get that it's natural for Polish, but without context for a learner who's never seen this before, it's confusing.
E.g. I thought this meant "It always comes back to this book" - meaning this book comes up again and again, it keeps being relevant, it's the topic of conversation, the source of the problem, etc. - which I now see is completely wrong.
I think we should learn natural Polish and these phrases should be included, but please give a heads up for things like this in the lesson tips. It makes no sense for an English speaker.
Actually the only thing wrong in "It always comes back to this book" is "always", which should have been "often". We accept "It", it could potentially work, for example "The discussion often goes back to this book".
Well... wasn't it explained in the lesson tips at the very beginning? We don't really repeat the same information in the tips...
Ack! I meant often, my mistake. Good to know "it" is accepted in the sense I mentioned, thanks.
I didn't remember it being explained before but I went back and found it (if anyone else needs to check, it's in the Basics 2 section right at the bottom of the tips). Sorry to doubt you.
We had a long discussion about it, but decided to reject it after all. It's the exact same word order as in Polish, but the emphasis is different. It also seems to me that 'often' in the beginning suggests the meaning 'it is not unusual for him', which the Polish sentence does not imply.
We don't want you to think that those two word orders (EN&PL) are in any way equivalent.