When the two halves sit on different timelines
In the two conditionals you already know, condition and result sit on the same timeline. In conversation that often does not fit: I didn't go to university back then, so I have no degree today. The condition is in the past, the result in the present. That is what the periodo ipotetico misto is for.
English does it the same way: “If I had studied, I would have a degree now.” No new forms are needed, only a new combination.
Condition back then, result today
This is the more frequent direction. Something did not happen back then, and that is why things are different now from how they could be.
The main clause takes the condizionale presente, because the result holds now, which is where English says would be rather than would have been. Words like ora, adesso or oggi make this explicit, but they are not obligatory.
Se avessi studiato, ora sarei laureato.
If I had studied, I would have a degree now.
Se non avessi lasciato quel lavoro, adesso guadagnerei il doppio.
If I hadn't left that job, I would be earning twice as much now.
Se fossimo rimasti a Milano, i bambini andrebbero a scuola lì.
If we had stayed in Milan, the children would go to school there.
State today, result back then
The other direction is rarer. Something that holds today, or is lasting, would have caused something in the past, but it does not, because it is not the case.
The se-clause names a state or quality that holds today: fossi paziente, avessi la patente, abitassimo vicino. That takes the congiuntivo imperfetto. The result would have fallen in the past, so the main clause takes the condizionale passato.
Se fossi più paziente, non avrei litigato con lui ieri.
If I were more patient, I wouldn't have argued with him yesterday.
Se avessi la patente, ti sarei venuto a prendere.
If I had a driving licence, I would have come to pick you up.
Se abitassimo più vicino, saremmo passati a salutarvi.
If we lived closer, we would have dropped by to say hello.
