Interrogative Pronouns (wer, was, wem, wen, wessen)
How do "wer" (who) and "was" (what) decline?
Nominative: wer (who), was (what)
Accusative: wen (whom), was (what)
Dative: wem (to whom), – (no dative for was)
Genitive: wessen (whose), – (no genitive for was)
Example
Wer kommt? (Who is coming? - Nominative)
Wen siehst du? (Whom do you see? - Accusative)
Wem gibst du das Buch? (To whom do you give the book? - Dative)
Wessen Buch ist das? (Whose book is that? - Genitive)
Was machst du? (What are you doing? - Accusative)