si (condition)

Example

Si tu viens demain, nous irons au musée.

Do not write

Si tu viennes demain, nous irons au musée.

Why

AN ABSOLUTE RULE: after conditional si, NEVER the subjunctive, and never the future or the conditional either. The three patterns are si + present, si + imperfect, si + pluperfect. This is one of the few French rules with no exception at all.