Please clarify when to use will vs. would?

I understand the construction: If X happen(s), then Y will happen. And, I understand that "would" is used in hypothetical situations.

What about: If X happen(ed), then Y will/would happen. (I think would is correct here, but can I make the generalization that if an event already occurred than "would" should be used?)

Thanks again for all your help.

There are a lot of possible variations here; it depends in part on the meaning of the sentence. For instance, both of these are correct:
If it rained yesterday, then I used my umbrella.
If my car died, I would be unhappy.
If my car dies, I will be unhappy.

The first is referring to an actual event and is set up as an if-then statement; the second is a past-tense hypothetical; the third is a present-tense possibility. In the hypothetical circumstance, yes, I use "would" not "will." But note that the generalization is not that the event occurred, in the second case. The generalization is that if I use a past-tense hypothetical, then I use "would."