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 sabÃa. If they left, no one knew.
Si ella comÃa poco, era porque querÃa 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, viajarÃa mucho más. If I had more money, I would travel a lot more.
Si pudiéramos, comprarÃamos una casa. If we could, we would buy a house.
Si no fuera por mi amigo, yo no irÃa. 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, habrÃamos podido salir ya. If you had arrived on time we would have been able to leave already.
Si yo hubiera preparado más comida, habrÃas podido invitar a tus amigos. If I had prepared more food you would have been able to invite your friends.