This a test question from Jänich:
Let $M \neq\emptyset$ (a smooth manifold) and $1 \leq k \leq n=\dim M$. Can there exist a map $f:M\to M$ with the property that $f^*\omega=-\omega$ for all $\omega \in \Omega^kM$?
The answer key says NO, NEVER! But I don't know why, this is what I get if I suppose yes:
Since this is must be true for all $\omega \in \Omega^kM$, I could reach a contradiction by using a particular $\omega$, but who?