French "if" sentences come in three flavors, real/possible, hypothetical, and contrary-to-fact past, and each one locks si and the result clause into a specific pair of tenses that doesn't mix and match.
1. Real or possible: si + présent, futur
For something that could realistically happen.
| Si tu étudies, tu réussiras. | If you study, you'll succeed. |
| Si j'ai le temps, je t'appellerai. | If I have time, I'll call you. |
2. Hypothetical: si + imparfait, conditionnel présent
For something unlikely, or purely imagined, in the present or future.
| Si j'avais de l'argent, je voyagerais. | If I had money, I would travel. |
| Si tu étais moi, que ferais-tu ? | If you were me, what would you do? |
3. Contrary-to-fact past: si + plus-que-parfait, conditionnel passé
For something that didn't actually happen, looking back and imagining a different outcome.
| Si j'avais su, je serais venu. | If I had known, I would have come. |
| Si elle avait étudié, elle aurait réussi. | If she had studied, she would have succeeded. |
4. Watch out: never futur or conditionnel right after si
Common mistake: si is never directly followed by a futur or a conditionnel, only présent, imparfait, or plus-que-parfait. "Si tu viendras" is wrong, it has to be Si tu viens (présent), even though the result clause can be in the futur.
5. Quick reference
| si + présent | → futur (real/possible) |
| si + imparfait | → conditionnel présent (hypothetical) |
| si + plus-que-parfait | → conditionnel passé (contrary-to-fact, past) |