Using the Imperfect or Past Subjunctive with Conditional
Talking about hypothetical situations, or situations that are contrary to fact, requires some rather complicated grammar. In Spanish, if you are speaking about hypothetical situations for the present time you might want to use the following pattern:
Si + imperfect subjunctive + conditional
Conditional + si + imperfect subjunctive
Si yo fuera el presidente ayudaría a los niños de la calle.
If I were the president I would help the children who live on the street.
Yo viajaría por todo el mundo si fuera muy rico.
I would travel around the world if I were really rich.
Another common construction which uses the imperfect subjunctive is the phrase como si (as if) to express a situation that is contrary to fact.
Hablas como si fueras el presidente de la compañía.
You speak as if you were the president of the company.
Actúas como si estuvieras culpable.
You act as if you were guilty.