What “mixed” means
Normally condition and consequence sit in the same time frame: both in the present or both in the past. In a mixed conditional they sit in different ones. English does exactly the same (If I had studied medicine, I'd be a doctor now), and each half follows its own time frame.
Past condition, present consequence
Something did (or didn't) happen back then and you are living with the result today: English if I had…, I would…. A word like ahora often shows that the consequence is in the present.
Si hubiera estudiado medicina, ahora sería médico.
If I had studied medicine, I would be a doctor now.
Si no hubiera comido tanto, no estaría tan lleno.
If I hadn't eaten so much, I wouldn't be this full.
Si hubieras escuchado, sabrías la respuesta.
If you had listened, you would know the answer.
Present state, past consequence
A state or trait that holds today explains a past action: English if I were…, I would have…. English hides the subjunctive in were and knew; Spanish uses the imperfecto de subjuntivo here (fuera, tuviera, supiera).
Si fuera rico, habría comprado esa casa.
If I were rich, I would have bought that house.
Si supiera conducir, habría ido en coche.
If I knew how to drive, I would have gone by car.
Si no fuera tan tímido, le habría hablado.
If I weren't so shy, I would have spoken to her.
