There are different types of si clauses, but they all have two things in common:
1) The English result clause might be preceded by "then," but there is no equivalent word preceding the French result clause.
Si tu conduis, je paierai.
If you drive, (then) I'll pay.
2) The clauses can be in either order, si clause followed by result clause, or result clause followed by si clause, just so long as the verb forms are paired correctly and si (if) is placed in front of the condition.
Je paierai si tu conduis.
I'll pay if you drive.
Types of si clauses
Si clauses are divided into types based on the likeliness of what is stated in the result clause: what does, will, would, or would have happened if.... The first verb form listed for each type names the condition upon which depends the result, which is indicated by the second verb form.
- First conditional: Likely / Potentiel
Present or present perfect + present, future, or imperative
- Second conditional: Unlikely / Irréel du présent
Imperfect + conditional
- Third conditional: Impossible / Irréel du passé
Pluperfect + conditional perfect
Test on si clauses
*Please note that the term "conditional" here refers to the condition being named; it does not mean that the conditional mood is necessarily used in the conditional sentence. As you can see above, the conditional mood is not used in the first conditional, and even in the second and third conditional, the conditional mood does not name the condition, but rather the result.
**Rules about the sequence of tenses are explained in more detail in this lesson: concordance des temps