Translation:If I were you I would not be using this phone.
Is there any reason that this sentence shouldn't be "I wouldn't use this phone if I were you." The answer given seems a bit unnatural (the tense seems odd given the situation.)
I wonder if it is understood that the use of Present Continuous here can only mean that the person is already using the phone. Is that what the Polish sentence really means?
After "If I were you" we nearly always use "I would/wouldn't". I'm afraid this use of Continuous doesn't clarify anything for me; it just confuses me as to the real meaning of the Polish.
Some random (and slightly lengthy) thoughts on the use of "I wouldn't be using".
"If I were you ..." constitutes the condition clause of what in the EFL/ESL world we call a Second Conditional (see Note 1), with the standard pattern having "would" or "could" in the result clause:
"If" + Past Simple ..., ..."would, could"
"If I were you, I wouldn't use that phone"
This could refer to a single action, or repeated actions, depending on context. Of itself it has no meaning of being "perfected" or not. Although the simple form is by far the most common here, it is also occasionally occurs in the continuous, and in an affirmative sentence it could well refer to future actions, perhaps imperfective in nature:
"If you got the job, what would you be doing?"
If, however, we use the continuous form in the negative version of this construction, it almost always suggests that the action is already in progress. This is neatly illustrated in a caption somebody has added to a photo of Alain Delon photographing a pigeon sitting on top of his vertically raised camera:
"If I was him, I wouldn't be doing that with my mouth open!"
The few examples (see Note 2) I've found on the web of "wouldn't be using" don't really give enough information to swing the argument one way or t'other, but I checked out a couple on forums, and sure enough, they both refer to events already happening:
"Actually if I were you, I wouldn't be using an app that locates you in an area at all."
(the poster of the question was already using the app)
"And again, if I were you I wouldn't be using that product without knowing what's in it"
(the poster of the question was already using the product)
Of the nine examples I found of "If I were you, I wouldn't be doing ..." (see Note 3), all appear to refer to an action in progress, except for one, which admittedly does refer to repetition in the future (see Note 4), but one swallow doesn't make a summer.
One useful website aggregates examples of "wouldn't be doing" from the media in the States and the UK. Over twenty of the examples (the majority) are Second Conditionals, and as far as I can see, all of these refer to actions already in progress.
As I understand it, the idea of Duolingo is that normal people (who tend not to be grammar freaks) can learn a language by translating simple sentences, without having to have lengthy grammar explanations, and I suspect that the majority of users never even look at the comments pages.
My guess is that when most native speakers see this sentence, they're going to think that it refers to an action in progress, after they've got over the initial shock of seeing a somewhat unusual verb form, that is. They then might well assume that the same is true of "nie używałbym". And while I understand that this is possible, my Polish students tell me that it would probably more often be referring to events in the future. Incidentally, I've been trying the Polish sentence out on my advanced students this week, and without exception they translated as "If I were you, I wouldn't use that phone".
So, while Past Continuous and Future Continuous are often quite a good fit to Imperfective, it's a false analogy, in my opinion, to extrapolate this to other Continuous forms, especially modals, which behave in a different way to normal tenses. Especially when the Simple form is by far the more common here, and when it could as well have an Imperfective sense as a Perfective one.
(1) aka Present Counter-factual Conditional, Present Hypothetical Conditional, Irrealis, etc
(2) There were about sixteen examples of "If I were you I wouldn't be using" in Google, compared with over 100 (confirmed) for "If I were you I wouldn't use".
(3) Compared with just over 120 for "If I were you I wouldn't do".
(4) "If I were you I wouldn't do any long runs after Sept 7th"