Translation:We swim every Monday and Friday.
Because ‘jeden Montag und Freitag/every Monday and Friday’ unambiguously describes a habitual action, which cannot be expressed with present continuous in English.
If you use 'every' with the present continuous, it's clear. "We are swimming every Monday and Friday." should be acceptable. This indicates the current state of affairs, allowing some room for it to change. "We swim Mondays and Fridays." would be plainer: a statement of fact without implications. "We swim every Monday and Friday." implies that you never miss those days; it's emphatic.
Exactly. Time adverbials are often expressed with a bare accusative in German.
I suppose it's short for jeden Montag und jeden Freitag.
A bit like "I have a knife and fork" being short for "I have a knife and a fork", with singular "a" before the seeming plural "knife and fork".
Shouldn't "We are swimming every Monday and Friday." also be accepted? Thanks!
In my opinion, no: since this is a repeated or habitual event, the present simple (we swim) should be used, not the present continuous (we are swimming).
My answer 'We go swimming on Mondays and Fridays.' is CORRECT. Why? Because in British English, at least, when you add an 's' to the days of the week in a statement in the Present Simple tense, it indicates an action that is repeated every week. So, one has the choice of saying 'on Mondays and Fridays' OR 'every Monday and Friday.' It means the same thing.
So are you asking why your answer is correct or why it was marked incorrect but should be right? I assume you mean the latter.
For one thing, Duo simply can't keep track of every possible translation of a sentence. Each translation has to be specifically added by a human, after all. So it's usually safest to translate fairly literally on Duolingo to ensure that your answer is accepted.
For another, I'd say the emphasis is actually a bit different on your answer, even if it basically means the same thing. "Every Monday and Friday" puts emphasis on "every"-- that this happens on all weeks, not just sporadically or every other week or something. "On Mondays and Fridays" leaves those possibilities open, even if the implication is probably that it's every week. So I'd say that the literal "Every Monday and Friday" is better