Si clauses: which tense?

In French, the tense after si (if) depends on how likely the condition is. The if-clause never takes the future or the conditional; those tenses belong only in the main clause.

• si + présent → futur, présent or imperative: Si tu viens, nous irons au cinéma. (If you come, we'll go to the cinema.) Si tu as froid, mets un pull. (If you're cold, put on a jumper.)
• si + imparfait → conditionnel présent: Si j'étais riche, je voyagerais. (If I were rich, I would travel.)
• si + plus-que-parfait → conditionnel passé: Si j'avais su, je serais venu. (If I had known, I would have come.)

Never say si j'aurais or si je serai for "if" (only si meaning "whether" allows them). Before il/ils, si becomes s'il/s'ils. The clauses can swap places without changing the tenses: Je t'appellerai si j'ai le temps.