When if-adverb clauses have 'future auxiliary-will'?

[bright_sun17]

It is gramatically right that If-adverb clauses have 'present verb' instead of future auxiliary 'will,'. like this.
If it rains tomorrow, we won't go on a picnic. ( o )
If it will rain tomorrow, we won't go on a picnic. ( x )
However, I often see 'if ' adverb clauses that have 'will'.
for example,
I shall be glad to go, if you will accompany me.
I'd like to know when and why 'if' adverb clauses have 'will'. I'd like you to take some more examples.
Thank you so much in advance. ^^

[RonBee]

You can say:

If it does rain tomorrow, we won't go on a picnic.

As for the other thing, I'm working on it.

[rewboss]

We use "will" in the following cases:

1. To make a prediction (I think Barack Obama will be the next US President).

2. When making a decision (I have no eggs -- I will buy some more).

3. When making a promise (I will write to her tomorrow).

Now, we can say "It will rain tomorrow", because that's a prediction. But when we're talking about a condition, a condition cannot at the same time be a prediction. "If it rains tomorrow" can't be a prediction because it's a condition.

But your other example is a promise: "If you will accompany me" means the same as "If you promise to accompany me". A promise can be a condition.