When is "will" used in an "if" clause?

There seem to be several questions here. I’ll answer the one in the title.

I believe foreign learners are taught, at least at the lower levels, that an if clause cannot contain the modal verb will . This is good advice as far as it goes. We have to say If you run, you will catch the train and not If you will run, you will catch the train. However . . .

Will (’ll) and won’t may be used in the if clause to express the possibility of willingness (or its absence) on the part the person addressed. It can be used as a form of politeness, as in If you ’ll come this way, I’ll show you a few samples. In its negative form it can be a threat, as in If you won’t try harder, I’m not going to help you.

As I often say, this topic seems to come up with some frequency here.

The answer lies in the different Deontic 'be willing to (do)' and Epistemic 'be expected to (occur)' senses that will has; only the Deontic sense is allowed in If-clauses.

Every modal auxiliary verb has at least one Epistemic and one Deontic sense, and they often have very different grammar and meaning, especially in hypothetical situations.