I'll have let you know if I think I'm going to be late.
I'll LET you know...
If you'll have been there before, you'll know how busy it gets.
If you HAVE BEEN there...
If everything goes well, we'll have been finishing by Friday.
...we'll have FINISHED by...
Let me know if you'll need any help with anything.
...if you NEED help...
We're going to miss the start if we won't leave soon.
...if we DON'T LEAVE soon.
If I'll get a bonus this year, I'm going to go to Hawaii.
If I GET a bonus...
Don't go to work if you won't be feeling well.
...if you AREN'T feeling well.
Could I borrow that book if you won't be using it?
...if you AREN'T using it?