What does como si mean?
Como si means “as if” or “as though” and introduces an unreal comparison: the clause does not describe reality, only an appearance. Someone who acts as if they knew everything does not in fact know everything. Careful English says as if he were a millionaire, not was: that were is the last surviving English subjunctive. Spanish does the same with every verb.
Same time or earlier
Which time level?
como si + imperfecto de subjuntivo
The pretence holds at the same time as the main action, or is generally true.
Gasta dinero como si fuera millonario.
He spends money as if he were a millionaire.
He is not a millionaire.
Me mira como si no me conociera.
He looks at me as if he didn't know me.
como si + pluscuamperfecto de subjuntivo
The pretence lies before the main action: something that had supposedly already happened.
Actúa como si no hubiera pasado nada.
He acts as if nothing had happened.
Habla español como si hubiera vivido en España.
He speaks Spanish as if he had lived in Spain.
The tense of the main clause does not matter: even after the present, the subjunctive is the past one, just like English “he talks as if he knew”. Only the main verb changes.
Habla como si lo supiera todo.
He talks as if he knew everything.
Presente
Habló como si lo supiera todo.
He spoke as if he knew everything.
Indefinido
Related phrases
Igual que si and lo mismo que si (just as if) work like como si. Fixed and without a verb: como si nada (as if it were nothing).
Lo dijo igual que si fuera verdad.
He said it just as if it were true.
Sigue trabajando como si nada.
He keeps working as if it were nothing.
