Is it written “se” or “sé”?
Both forms are correct.
Se, without the accent, it is a hypothetical conjunction which serves to introduce a hypothesis or a condition.
Se avessi tempo, andrei a fare una passeggiata al mare.
Se fossi ricco, comprerei uno yacht.
Se tu mi dessi attenzione, ti spiegherei cosa è successo.
Sé,with the accent, it is instead the third person singular reflexive pronoun.
Marco pensa solo a sé stesso.
Chi fa da sé fa per tre.
Le critiche di questo genere può tenersele per sé.
As it can be seen from the first example, in the locution sé stesso, “se” requires the accent as it performs the function of a reflexive pronoun.
The two words are homonymous that is they have the same form and are pronounced the same way but have a different meaning. In Italian, the accent is used to distinguish the meaning of two homonymous words.
