Deda, I think it has to do with the second clause being conditional on the first, while there is no prior condition for the first clause. But I could be wrong. I also think that there is an "archaic" formation in which "had" would be used instead of "would have". But I could be even more wrong there. These are the problems we run into when we try to apply Latin rules of grammar to an English that's lost most of its inflections and much of its Latin-based structure. Subjunctive? We don't need no steenkeeng subjunctive! But I don't do this for a living either, and would be interested in hearing from a real expert.

Aloha,
Mark