Questions & Answers about Lintu lentää puuhun.
Puu is the basic (dictionary) form meaning tree.
Puuhun is puu in the illative case, which is one of the Finnish “local” cases. The illative roughly means “into” or “to (inside)”.
So:
- puu = tree
- puuhun = into the tree
In this sentence, the bird is moving into the tree, not just being in or at the tree, so Finnish marks that idea of movement into directly on the noun with the ending -hun.