Si clauses in Spanish
A si clause in Spanish expresses a condition that must be met before something else can occur. The verb tense that follows the si clause depends on the likelihood of that condition actually happening. If clauses may express conditions that are probable, slightly probable or that are contrary to to a reality in the past or present.
If it refers to a probable condition, then indicative / indicative
Si me llamas, iré contigo al concierto. If you call, I’ll go with you to the concert.
Si quieren, les ayudo con su tarea. If they want I’ll help them with their homework.
Si ellos salieron, nadie sabía. If they left, no one knew.
Si ella comía poco, era porque quería bajar de peso. If she was eating a little it’s because she wanted to lose weight.
If it refers to a slightly probably condition or a situation contrary to reality in the present, then imperfect subjunctive / conditional
Si tuviera más dinero, viajaría mucho más. If I had more money, I would travel a lot more.
Si pudiéramos, compraríamos una casa. If we could, we would buy a house.
Si no fuera por mi amigo, yo no iría. If it weren’t for my friend, I wouldn’t go.
If it refers to a slightly probable condition or a situation contrary to reality in the past, then pluscuamperfecto of the subjunctive / conditional perfect
Si Uds. hubieran llegado a tiempo, habríamos podido salir ya. If you had arrived on time we would have been able to leave already.
Si yo hubiera preparado más comida, habrías podido invitar a tus amigos. If I had prepared more food you would have been able to invite your friends.