One,
Si+ present tense (clause part), present tense/general future tense/imperative sentence (main clause part).
It means that present or future assumptions may be realized. Another point that needs special attention is that "si" will not be used with the general future tense, that is, the clause after "si" will not use the general future tense.
Sentence:
S'il fait beau (present tense), on va à la mer (present tense). If the weather is fine, we will go to the seaside.
Sielle Viet (present tense), on partira ensemble (simple future tense). If she comes, we will leave together.
Si tu es d'accord (present tense), appelle-moi (imperative). Call me if you agree.
Si+ imperfect past tense (clause part), conditional present tense (main clause part)
Can express assumptions about the present or the future. But the possibility of realization is very small, or contrary to the facts.
Second,
Sij'étais à ta place (incomplete past tense)
Sentence:
The condition is present tense. If I were you, I would buy this book. (The present tense of the condition means contrary to the current actual situation)
He is a failed lover because he has a bad temper. If the weather is fine, we will have a picnic. (The conditional present tense indicates possible actions)
Third,
Si+ past tense (clause part) and conditional past tense (main clause part). Can express assumptions about the past. Indicates a situation contrary to past facts.
Sentence:
S'il avait fait beau (past tense), Jesse goes to the party on weekends. If it had been fine then, I would have gone to spend the weekend. Conditional past tense means that contrary to the past, we should have gone, but we didn't.
The same Chinese meaning, if "si" is used in simple sentences, its connotation will be different.
Si+ indicates suggestions or wishes in the past unfinished.
How are you doing in China? Shall we go to the movies?
We're in this together. ! If only we had gone for a walk!
Si+ is finished, expressing regret.
I wish I knew.