The constructions of these hypothetical statements follow very strict tense rules in French (they are actually the same as in English). I suggest you remember the examples I use to illustrate these rules and different kind of hypotheses.
1 – hypothesis that CAN become real – (called Hypothesis on the future)
Use this construction if you are talking about something that is happening in the future, or right now, but with a strong chance of it becoming a reality = full hope
Si + present / future
Demain, s’il pleut, tu iras au musée, et s’il fait beau, tu iras à la plage (we all know how hypothetical the weather forecast is….)
Tomorrow, if it rains, you will go to the museum, and if it’s nice out, you will go to the beach.
This hypothesis CAN become a reality. The conditions you are talking about are in the future, they might or might not become a reality.
This is the hypothesis parents use all the time with children… “Si tu manges toute ta viande, tu auras une glace” (If you eat all your meat you’ll have some ice cream.)
Instead of the simple future, we also use the near future construction: aller + infinitive.
Si tu manges trop de glace, tu vas avoir mal au ventre.
If you eat too much ice-cream, you’re going to have a tummy ache (I believe English speakers would say will here, to reinforce the certainty of it. In French, it’s the contrary, we’d use the near future to reinforce the likeliness of it to become a reality).
2 – Alternative present reality (called hypothesis on the present)
Use this construction to express an alternative present reality. You are in a certain situation, and you are saying what you would do if it was not so.
Si + imperfect /present conditional
S’il faisait beau maintenant, tu irais à la plage (but it’s raining now)
If it was nice out now, you would go to the beach.
The statements for this sort of hypothesis often end with a “but + stating the real situation”.
If she was rich, she would buy a boat (but she is not rich…)
This hypothesis CANNOT become a reality right now. However it often carries a notion of wish: she is not rich, but maybe one day she’ll be rich. It’s raining now, but maybe it will clear up and then you’ll go to the beach.
3 – Alternative past reality (called hypothesis on the past)
Use this construction if you are talking about something that happened before, that has ended now = there is no more hope = expressing a regret, bitching about what did not happen but you wished did.
Si + pluperfect / past conditional
Hier, s’il avait fait beau, tu serais allé(e) à la plage (but it rained all day…)
Yesterday, if it had been nice out, you would have gone to the beach.
This hypothesis CANNOT become a reality at all. You are talking about a past event. You are saying what you would have done if the conditions had not been what they were.
The pluperfect follows the same agreement rules as passé composé, but with être / avoir in imperfect.
The past conditional follows the same agreement rules as passé composé, but with être / avoir in conditional present.
Check out my verb drills to train on these tenses.
You can also reverse the order of the sentence: “You would have gone to the beach if the weather had been nice.”
4 – Si for “when”
I am not sure whether it is grammatically correct, but we use it a lot in spoken French.
Si + present / present
Si je gagne au loto, j’achète une voiture. If/when I win the lottery, I am buying a car.
The concept here is the instant reaction. It is no longer a hypothesis on what I will do, since what I will do is a certainty once the first condition happens – hence the same time line… It makes the sentence much more dynamic than si + present /future.
If you liked my grammar lesson on the constructions using si in French, you might also like: