[if past simple + will] is used when you are not just talking about some possible situation, but about a situation that has really happened (at least in part) or will very, very likely happen. This is more of a "feeling" thing than a hard-and-fast rule. Think about these sentences:
- If he found out, we will have to explain it
- If he found out, we would have to explain it
- If he were to find out, we would have to explain it
To my (native American English speaking) ear, these move from most likely to least likely. The first one suggests that there is a good chance that he HAS found out (my feeling is 50-50), and that's what will have to happen if it is true that he did find out. The second feels more as though it is only theoretical (maybe <25%? but that's just a random number)-- it's not likely that he would find out but in the unlikely event that it happened, we would have to explain. The third one sounds even more distant as a possibility -- probably because the "if he were to" pattern is not as commonly used in casual speech.