"Tuesday is not my day."
Translation:Wtorek to nie mój dzień.
if you use "to", you can use nominative, if you use jest without "to", you need to use instrumental. Wtorek nie jest moim dniem.
negation - if a verb needs object in accusative, negated verb needs object in genitive. all other cases stay the same. accusative after preposition stays.
In this case "to" is used instead of "jest". You can use this words interchangeably. Your sentence has a tiny mistake. It should be " Wtorek nie jest moim dniem".
Why can't we say "Wtorek nie to mój dzień"? Does "to" have to be after Wtorek?
When you have "to" used like this, it kinda has a hidden "jest". "Wtorek to nie (jest) mój dzień".
And then you can see easier, that you in fact negate 'being my day' - so it's correct. In your sentence, it's hard to even say what you negate... but it doesn't work.
My answer, "Wtorek nie to mojdzien"--I didn't "use the worng word;" I used the wrong order.