How to make demonymic forms for cities/areas etc.
*this is a common question about Serbian, Bosnian, Croatian and Montenegrin

Zdravo. I recently learned some demonyms like;

Beograđanin
Zagrepčanin
Dalmoš
Evropljanin
Mostarac
Sarajalija
Bišćan
Kotoran

There seem to be various ways to make demonyms.
Are there any specific rules to choose a proper suffix depends on the name of cities/areas/countries?
If no, should I learn them one by one?

Question

How to make demonymic forms for cities/areas etc.
*this is a common question about Serbian, Bosnian, Croatian and Montenegrin

Zdravo. I recently learned some demonyms like;

Beograđanin
Zagrepčanin
Dalmoš
Evropljanin
Mostarac
Sarajalija
Bišćan
Kotoran

There seem to be various ways to make demonyms.
Are there any specific rules to choose a proper suffix depends on the name of cities/areas/countries?
If no, should I learn them one by one?

Answer

Hi Jeziko. I am not a language specialist and that is why I am going to help you using my experience. Demonymic forms generally come with the female suffix ANKA and the male one with the ANIN. Unfortunately there are a lot of exception about which even the language specialists can not make a common agreement. 
Your examples:
Beograđanin/Beograđanka
Zagrepčanin/Zagrepčanka
Dalmoš (It is nick name). Dalmatinac/Dalmatinka
Evropljanin/Evropljanka
Mostarac/Mostarka
Sarajalija/Sarajka
Bišćan (Not common). Bišćanin/Bišćanka 
Kotoran (Not common). Kotoranin/Kotoranka 
Some more examples:
Valjevac/Valjevka. (City of Valjevo, SRB).
Zagorac/Zagorka, (Region in CRO).
Istrijan/Istrijanka, (Region in CRO).
Tuzlak/Tuzlanka, (City of Tuzla, BiH).
Vukovarac/Vukovarka, (City of Vukovar, CRO).
Hercegovac/Hercegovka, (Region BiH)...

Good luck !