First-order logic - Does is exist a sentence that is satisfiable by any finite models? Well, i try to found an example of sentence $\Psi$ which satisfiable by any finite models, but there exist some infinite model that doesn't satisfies it.
We can choose any language we want, no restrictions.  
So far i only think about a sentence that say that in finite model there must be a maximum. But, how to write it in a way that in every model (that can give any ןnterpretation it want to the language) it will say this?
tnx!

Let $\psi$ be in the language $L=\{ f\}$ where $f$ is a unary function symbol. 
Then write "$f$ is an injection $\to$ $f$ is a surjection". 
Show that this will hold on any finite structure, but you can clearly find an infinite counter example
