Learn Hardcore Greek: Αν συμπεριφέρεσαι ήρεμα, ακόμα και μια δύσκολη συζήτηση γίνεται πιο εύκολη. - If you behave calmly, even a difficult discussion becomes easier.

In this sentence, the condition is a general truth or usual result, not a one-time future event. Greek often uses the present tense in both parts of this kind of sentence:

Αν συμπεριφέρεσαι ήρεμα, ...
... γίνεται πιο εύκολη.

So the sense is: Whenever / if you behave calmly, the conversation becomes easier.

Why is it συμπεριφέρεσαι and not a more active-looking form?

The verb is συμπεριφέρομαι, which means I behave. It looks like a middle/passive verb in form, but its meaning here is active.

So:

συμπεριφέρομαι = I behave
συμπεριφέρεσαι = you behave
συμπεριφέρεται = he/she/it behaves

This is very common in Greek: some verbs use middle/passive endings but have an active meaning in English.

What exactly does συμπεριφέρεσαι mean here?

It means you behave or you conduct yourself.

So:

συμπεριφέρεσαι ήρεμα = you behave calmly
more literally: you conduct yourself in a calm way

It is about how someone acts during an interaction, not just how they feel internally.