"Which man? This man!"
Translation:Welcher Mann? Dieser Mann!
If you put the demonstrative pronoun together with the noun it refers to, they have to match in number, case and gender.
- Dies/Das ist ein schöner Mann. (no match)
- Dies/Das sind schöne Männer. (no match)
- Dies/Das ist ein blaues Auto. (match, Auto is neutral by chance)
- Dieser Mann ist schön. (match)
- Diese Männer sind schön. (match)
- Dieses Auto ist blau. (match, n.b. it's not dies Auto but dieses Auto)
For the first two sentences I use a neutral singular pronoun, because I introduce something "unknown". Like the adjectives that I used, everything that is directly in front of the noun that it modifies, has to match.
I don't know how old this is, the app won't tell me. But I think the accusative means that the subject in the accusative is being acted on by a verb. In this sentence, there is no verb acting on the subject. E.g. "Der Hund singt zu diesen Mann."
(If that is wrong, please someone correct me, but for the purposes of demonstrating the use of diesen I think it's okay.)
In that sentence, "diesen Mann" is being 'verbed' by the dog, who is singing to the man.