I would do B if you could do A.
This is a statement which has been bothering me for quite a while. I come across such statements often and, to me, they make no sense. Could is the subjunctive of can — it is often used to express possibility (or anything else along those lines). Sometimes, could is also used as the "past form of can". Now, the second conditional is used to express unreal(istic) situations in the present, we construct it using the simple past form of the verb at hand and instigate a would in the "response sentence".
Now, does this could then indeed relate to the capability and not to possibility of someone doing A? And, furthermore, isn't the condition fulfilled only by the fact there exists this capability (or possibility)? That's the part that makes no sense for me.
You don't actually have to "do A", you just must have the possibility/capability to "do A", for the if clause to return true and make the speaker "do B".
I would do B if you did A.
does the proper job of the original idea that the speaker intended.
And since the second conditional indeed is used for unreal situations in the present, why do we so often use it to express conditions to other people?
Wouldn't it be more prudent to actually state a requirement in the present, which deals with "real situations":
I will do B if you do A.
What's the difference?