if I will

"If I will" is incorrect in English.

The reason is that "if" indicates a conditional situation whereas "will" indicates certainty. Therefore, they would contradict each other!

hirashinIn what case could you use "if I will"?

Not typically in direct questions, as your example shows.
________________

However, in indirect questions:

I don't know for sure if I will be available Wednesday.
I have no idea if I'll have time, but I'll try to meet with you then.
I wonder if I'll need an um

How about this one though it's not "if I will", but "if she will"?
I'll be happy if she will help me financially.

Do you think it is also incorrect?

Hirashin

Yes it is incorrect (for the reason I mentioned above).

You can say: "I'll be happy if she helps me financially."

Thanks for the help, teechr.

hirashinHow about this one though it's not "if I will", but "if she will"?

It doesn't matter what subject you use in the sentence. It's the same rule.

hirashinI'll be happy if she will help me financially.

No. You don't want this.

hirashinDo you think it is also incorrect?

I do. It is i

hirashinI'll be happy if she will help me financially.

I do not agree with what some others have said. I think this is possible and natural with the sense of '... if she is willing to help ...".

fivejedjon hirashinI'll be happy if she will help me financially.I do not agree with what some others have said. I think this is possible and natural with the sense of '... if she is willing to help ...".

Thanks for the help, fivejedjon. I'm relieved. Actually, the sentence is from one of the grammar books, which I trust.
I really appreciate your help.

'Will' indicating certainty is incompatible with 'if' indicating possibility. Thus, We will cancel the picnic tomorrow if it will rain is not possible. However, there is no problem when 'will' indicates willingness.

Thanks for your further help, five jedjon. That's the comment I've wanted.