¿Cómo se forma el tercer condicional?

El tercer condiciónal se refiere a hechos pasados contrarios a la realidad. El patrón general es: IF /UNLESS+ SUBJECT + PAST PERFECT , SUBJECT + WOULD HAVE + PAST PARTICIPLE Ejemplos: "If I had left earlier, I would have arrived at the airport in time to take the flight." (Si hubiera partido más temprano, habría llegado al aeropuerto a tiempo para tomar el vuelo.) = I didn't leave earlier so I lost the flight. "If I hadn't bought the tickets in advance, I would haven't seen the show." ("Si no hubiera comprado las entradas por adelantado, no habría visto el espectáculo.") = I bought the tickets in advance so I got to see the show. "Unless it had been strictly necessary, I wouldn't have asked him for help." (A no ser que hubiera sido estríctamente necesario, no le habría pedido ayuda.) Se puede colocar la proposición con "would" primero, en cuyo caso no se utiliza coma: "I would have arrive at the airport in time to take the flight if I had left earlier." Dependiendo del significado que se desee obtener, "WOULD" puede ser reemplazado por otro verbo defectivo/modal como "could" o "might".

Si hubiera aprobado el exámen, habría ido a la universidad. If I had passed the exam, I would have gone to university. If + had + past participle/ would (or could) + have + past participle

If + past perfect (en español sería plusc. de subjuntivo --> "si hubiera hecho tal cosa") y luego un conditional perfect (condicional perfecto en español --> "habría...") Cheers!

To make a sentence in the third conditional, we use, If + past perfect, would/wouldn't have + past participle. If you had told me about the meeting, I would have come. If you had told me about the meeting, I wouldn't have missed it.