Questions & Answers about Mañana voy a la playa, salvo si llueve.
Why is voy (present tense) used here instead of the future tense iré?
Why do we use the indicative llueve here instead of the subjunctive, since the rain is an uncertain future event?
In Spanish, the word si (if) is never followed by the present subjunctive in conditional clauses, even when talking about the uncertain future. Because this is a likely condition, you must use the present indicative: si llueve.
Could I use salvo que instead of salvo si?
Yes, but it changes the grammar. The phrase salvo que (unless or except that) always triggers the subjunctive because it introduces a hypothetical exception. If you use it, the sentence becomes: Mañana voy a la playa, salvo que llueva.