No, it doesn't. 'A big mouse' refers to some mouse which happens to be big. 'The big mouse' refers to one specific mouse.
For example, if you were talking about your pet mice you would say 'the big mouse' because you have a specific mouse in mind (i.e. your pet mouse). I hope this makes sense.
Not a real exception, just a rule you did not know.
Feminine nouns do not have to end with -a, there are many feminine nouns that end with a "soft" or "semi-soft" consonant.
I wrote a bit about genders in Polish https://www.duolingo.com/comment/14133935 and my opinion on those is :
If a word ends with ń ć, ś, ź, ż, sz, cz - you can toss a coin, or check a dictionary, it might be feminine or masculine.
About the word ending with -cz, you're saying it's 50/50 regarding the gender matter if i interpret well your coin metaphor.
Let's assume, as you do, there are only two genders (no neutral for example), is it really the case? In Czech, at least, it's almost always masculine.
Thus, his remark (saying it's an exception) would make sense.
What do you think?
immery hasn't been active for years now, but I think that you're right, the division is pretty unlikely to be 50/50. We'd need a lot of data to actually figure it out ;)
About those endings with 'acute accent' - if the word ends this way and it's an abstract notion (like 'przyjaźń' = friendship or 'miłość' = love), then I believe that (almost?) all of those are feminine.
There are Tips and Notes until some point, not for the whole course. More are coming. You can access them by clicking at a skill icon and then the lightbulb icon, if you don't see it in your app, use the website version.
Also, you may find a lot of interesting topics here: https://forum.duolingo.com/comment/16296174
There is an error with this "listening" question. When I am asked to type in Polish what I hear in Polish, i get the erroneous error "You typed the answer in English, not Poilish" despite having correctly responded "Duża mysz je ser". I don't see bow that can be construed as English.
Well, if the system 'screamed an error', then it suggests that your answer wasn't correct after all.
Your answer isn't connected in any way to your comment. We have no idea what you answered, so unless you tell us, we can't say anything about it. And even then there's a possibility that you just think you wrote that, but actually made some accidental typo that made the answer rejected, so the only surefire solution is to provide a report and/or a screenshot.