Real conditions: si tengo tiempo, voy

Explanation

Open, realistic conditions — things that may well happen — use si + present indicative in the if-clause, with the present, the future, or a command in the main clause: si llueve, no salimos; si firmas hoy, empezamos el lunes; si puedes, llámame.

A firm rule: never use the present subjunctive right after si. *Si tengas, *si vengas are impossible — it's si tengo, si vienes. (The subjunctive does appear in conditions, but only the imperfect/pluperfect kind, for unreal situations — the next rules.)

Si + a past indicative checks a real past fact, often to draw a conclusion: si lo dijo, fue por algo ('if he said it, there was a reason'). For hypothetical or contrary-to-fact conditions, the imperfect subjunctive + conditional takes over.

Examples

Related rules

• Unreal present: si tuviera, haría
• Lost pasts: si hubiera sabido, habría venido