Si hubiera sabido: all three si clauses, and como si

The three types on one screen

Type 3: unreal past

When the if is about a past that did not happen, use si + pluperfect subjunctive, then the conditional perfect:

In the result half, hubiera can replace habría. Both are standard:

• Si lo hubiéramos sabido, no hubiéramos venido. - If we had known, we wouldn't have come.

In the si half it never works the other way: never si habría sabido.

An unreal past with a result now

When the result is about now, it takes the plain conditional:

• Si yo hubiera estudiado, ahora tendría un buen trabajo. - If I had studied, I'd have a good job now.

Como si: always the past subjunctive

Como si (as if) compares with something imagined, so it takes the imperfect subjunctive, or the pluperfect for an earlier action:

• Habla como si fuera el jefe. - He talks as if he were the boss.
• Me trata como si no me conociera. - He/She treats me as if he/she didn't know me.
• Me miró como si hubiera visto un fantasma. - He/She looked at me as if he/she had seen a ghost.

Never the indicative: como si es el jefe is wrong.

Mistakes English speakers make