"Who is this?"
Translation:Kto to jest?
Why is it wrong to say "Kto jest to"? Can't "kto" and other question words be a replacement of the nouns itself or it doesn't work that way in Polish?
can't "kto" and other question words be a replacement of the nouns ? Yes they are.
Kto jest? is a correct sentence (who is ?)
but the sentence is "who is this?" so you need "to" in the sentence, but "to" in a pronoun, so we don't like to leave it at the end of sentence.
Cases. This is a very simple, basic sentence "Who is this?", so "who" is the subject and therefore uses Nominative: "kto".
"kogo" could be Accusative or Genitive, so for example "Kogo widzisz?" (Who/Whom do you see), as "widzieć" (to see) takes Accusative.