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 on the future = hypothesis that CAN become real
Si + present / future tense
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 - Hypothesis on the present = alternative present reality
Si + imperfect /conditional tense
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.
This hypothesis CANNOT become a reality right now. But it might change in the very near future - maybe the weather is going to clear up in 5 minutes...
This is the most commonly used hypothesis in French, it bears a notion of wish. For example, in a job application letter, I would say "Si je travaillais pour FrenchToday, je suis certaine que mon français s'améliorerait". (If I worked for FrenchToday, I'm sure my French would get better). Since it's a job application, there is no way this hypothesis applies to the present. We should use a hypothesis on the future construction. But I guess we use that one in the idea "I wish so much I worked there now, that would be my reality if I worked there now."
As before, instead of using the conditional, you will hear this construction with the "near future" in the past - I don't know if there is a better grammar name for it, but this one should be self explanatory... Note that to build the "near future" in the past, you put "aller" in the imperfect.
Tu avais décidé que s'il pleuvait, tu allais regarder la télé.
You had decided that if it rained, you were going to watch TV (English speakers here are more likely to say "you would watch TV".)
3 - Hypothesis on the past = alternative past reality
Si + pluperfect / past conditional tense
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.
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 tense / present tense
Si je gagne au loto, j'achète une voiture. If/when I win the lottery, I buy 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 tense/future tense.
If you liked my grammar lesson on the constructions using si in French, you might also like: