Conditionals

If you would do me a favour, I would repay you." Yes, it's correct. It's used very rare, generally in formal way. It shows a less probability and relates to the Future!

Why is it not: "If you did me a favour, I would repay you."? It's fine.

That's just what I thought. So would-would is used for future and not too likely conditionals, right?

So if you, maybe, some time in the future, would do me a favor, I would repay you, if that ever happens.

1. If you did me a favour, I would repay you.

2. If you would do me a favour, I would repay you.

I would agree that #2 relates to an event that has not yet occurred; while #1 expresses a general principle.

For me, the "would" in the if-clause in #2 has the sense "to be willing"; thus a paraphrase might be:

3. If you were willing to do me a favour, I would repay

Thank you, that makes sense.

If you would ..., I would ...
If I would ..., he would ...
If he would ..., she would ...
These "double would" constructions almost always involve bargaining, i.e., attempts to reach an agreement that benefits both parties.

CJ