Si clauses are used to introduce a hypothetical action that decides another action.
The tense used in the hypothetical situation decides the tense that follows.
Present tense - Present tense
S'il fait beau, je joue au tenis les lundis
If the weather's nice, I play tennis on Mondays
Present tense - Future tense
S'il fait beau demain, je jouerai au tenis
If the weather's nice tomorrow, I'll play tennis
Present tense - Imperative
S'il fait beau - joue au tenis!
If the weather's nice - play tennis!
Passé composé tense - Present tense
Si tu as fini le travail, tu peux partir
If you have finished your work, you can go
Passé composé tense - Future tense
Si tu as parlé avec lui, tu devras me raconter ce qu'il a dit ce soir
If you've spoken to him, you will have to tell me what he said tonight
Passé composé tense - Imperative
Si tu as parlé avec lui, dis-moi tout ce soir!
If you have spoken with him, tell me everything tonight!
Imperfect tense - Conditional tense
Si j'avais plus d'argent, j'achèterais une nouvelle voiture
If I had more money, I would buy a new car
Pluperfect tense - Conditional perfect tense
Si j'avais acheté mon billet plus tôt, il aurait coûté moins cher
If I had bought my ticket earlier, it would have been cheaper