Si + present, future
How to say what will happen if something else happens. French fixes the tenses in a way English does not, and the rule is absolute.
Si + présent , futur S'il pleut demain, je resterai à la maison.
| Si j'ai le temps, je viendrai. | If I have time, I will come. |
|---|---|
| Si tu veux, nous irons ensemble. | If you like, we'll go together. |
| S'il fait beau, on mangera dehors. | If it's fine, we'll eat outside. |
*Never put a future after si. Si il pleuvra does not exist — and neither does the English-looking si je serai. English says "if it rains" with a present too, so the trap is not the si* clause itself but the temptation to make both halves match.
The order can be reversed, and the comma disappears: Je resterai à la maison s'il pleut.
Two other patterns with si, both common and both without a future:
| si + présent, présent | S'il pleut, je reste à la maison. (general truth) |
|---|---|
| si + présent, impératif | S'il pleut, prends un parapluie. (instruction) |
Note the elision. Si becomes s' before il and ils only — s'il, s'ils — and never before elle or on: si elle, si on.
A third pattern, si + imparfait + conditionnel, is how you talk about things that are not true. That is week 23.
Knowing the rule is not the same as producing it.
This page explains the pattern. The track drills it: the point arrives inside a mini-dialogue, then production marked against a CEFR descriptor, then a module test that returns you to your two weakest sessions if you miss it.