Si j'avais su: the unreal now and the unreal past

Type 2: the unreal present

The condition is not true now: you are imagining. After si, the imparfait; in the result, the conditional.

• Si j'avais le temps, je viendrais. - If I had time, I would come. (I don't have time)
• Si tu étais riche, qu'est-ce que tu ferais ? - If you were rich, what would you do?
• Si elle parlait italien, elle travaillerait à Rome. - If she spoke Italian, she would work in Rome.

It is English if I had... I would... exactly. The imparfait here says nothing about the past: it marks the condition as unreal.

Type 3: the unreal past

The condition did not happen: you are rewriting the past, which makes this the pattern of regret. After si, the plus-que-parfait; in the result, the past conditional.

The past conditional is the conditional of avoir or être plus the past participle, with the same auxiliary as the passé composé:

Put the two halves together:

• Si j'avais su, je serais venu. - If I had known, I would have come.
• Si tu m'avais écouté, tu n'aurais pas fait cette erreur. - If you had listened to me, you would not have made that mistake.
• Si nous étions partis plus tôt, nous serions arrivés à l'heure. - If we had left earlier, we would have arrived on time.

Si j'avais su is worth learning whole: it is the everyday French for had I known. The full past conditional is on the conditionnel passé page.