You don't have to know the anatomy of the car to drive it.
Do not dwell on grammar, if you do not want to. Just try
to memorize the phrase "as is", and use it:
Mówię po polsku - I speak Polish (I know the language
and I know how to speak Polish when I need or want to)
Mówię po polsku - I am speaking in Polish (I actually
carry the conversation in this language as we speak)
Mówię po polsku/ po niemiecku/ po francusku/ po rosyjsku/ po
ukraińsku/ po włosku/ po hiszpańsku/ po grecku/po łacinie/ po węgiersku/ po szwedzku/ po chińsku/ po japońsku -
I speak Polish/ German/ French/ Russian / Ukrainian/ Italian / Spanish/Greek/Latin/Hungarian/ Swedish/ Chinese/ Japanese
"polski" is the adjective, sure, but I have no idea what "polsku" is. It's not any of the cases of "polski", I wouldn't also say that it's an adverb (they rather end with -o or -e)... I'd say that "polsko" is the adverb. "polsko brzmiący", "niemiecko brzmiący", "rosyjsko brzmiący" = 'Polish-sounding', 'German-sounding', 'Russian-sounding', could be interpreted in a way as "Polishly sounding".
As usual, I'm trying to understand Polish using Russian analogies. In Russian, "pol'skij" is the singular masculine adjective, like "pol'skij jazyk," but "po-pol'ski" is the adverb. Notice the j [й] at the end of the adjective and its lack thereof at the end of the adverb. Same goes for niemieckij/po-niemiecki, russkij/po-russki, etc..
There is another variant in Russian which might apply in Polish. The gender-neuter adjective pol'skoje changes to po pol'skomu when it's used as an adverb except in the cases of language "po-pol'ski". Maybe that's analogous to "polsku"?
Or maybe "polsko" changes to the locative/prepositional case "polsku" after "po."
Both expressions are correct translations:
Mówię po polsku - I speak Polish (I can speak / I am able to speak Polish) /
I speak in Polish/I am speaking in Polish (I carry the conversation in Polish)
The redundant personal pronoun "Ja" emphasizes the subject and shifts the attention from speaking the language to the person itself, which sounds like bragging or putting yourself in a better light by contrast with someone else:
Mówię po polsku - I speak Polish
Ja mówię po polsku - I ( and nobody else) speak Polish
Ja mówię po polsku, a ty/wy nie! - I speak Polish, and you don't!
I can't type Polish letters with my keyboard at the moment. I'll try to copy and paste it from a document. I'm talking about 'Polish E': Ę.
I seem to hear the woman pronouncing it as 'ehn' - you know, the way it is in Polish - when it's in the MIDDLE of a word. But when she says it at the end of a word, it seems like it's more like 'eh' or 'short e' in English. So in 'Ja mówię po polsku' I'm hearing 'eh' at the end, rather than that nasalized 'ehn.'
Is this just me, or does the value of Ę get reduced at the end (similar to the way 'd' or 'b' are devoiced at the ends of words in Polish)?
Thanks again. I've been hearing that in 'real life,' though I never really thought about it until now. Sometimes I do hear people giving Ę the full value at the end of a word, though it's rare. That's been 'public speakers' or rather people used to giving lectures, professors, that ilk. Then there's the Polish colleague who I would swear says something closer to 'tek' than 'tak', though she doesn't hear it..
By the way the 'a' in 'tak' doesn't doesn't actually sound anything like the 'u' in 'cut' to an English-speaker, though I know that to Poles it does, hence the graffiti that says '-ack off' rather than '-uck off.' If I followed the hint on Duolingo, an English-speaker would hear me saying 'tuck' rather than 'tak' - two clearly different sounds.
I think that Duolingo would do better to tell English-speakers that the 'a' in 'tak' sounds more like the 'o' 'doc' or 'a' in 'walk' than 'cut.' It's somewhere between 'talk' and 'tack' to an English-speaker, but it's not near the 'u' in 'cut.' It's a problem, because we want to put a diphthong in there, I know. And of course, Poles are always mispelling cat/cut, bat/but and so on because they can't really hear the difference, but it's very clear when a native-English-speaker makes those two sounds. The vowels in 'tak' and the English 'doc' or 'walk' are different, but closer than tak/cut. I can observe how my jaw drops when making the two different vowel sounds, as well as how my tongue moves differently with the two sounds.
It's hard to teach Poles to hear a difference between 'a' in 'tak' and 'u' in cut - I know because I spent 23 years teaching English here to Poles training to be English-teachers. But that doesn't mean that it transfers to English, since English-speakers hear very distinct sounds between 'u' in 'cut' and 'a' in 'tak.' We'll be saying something wrong, in fact, if we follow the advice to use the 'u' in 'cut.'
But you should bear in mind that "cat" is pronounced differently in British vs American English. It differs even within British English: https://en.wiktionary.org/wiki/cat#English. As you can see there are several different transcriptions and one of them is /kat/ in which /a/ sounds like Polish "a". "A" in "walk" sounds more like Polish "o" and to a Pole it would sound like "tok". If you pronounce it like in British "cat" or "cut", it will be good enough; although the difference may be audible.