e.g. Perhaps you will let me know as soon as you fix your travel plans.
e.g. I hope you will let me know when you get back from holiday.
e.g. It will be late when we arrive.
e.g. I'll come round after I finish my homework.
e.g. If I am in your city I will visit you.
The important thing to know here is that these are special 'word groups' which you need to learn.
if you have 'if' or 'whether' or 'as soon as' or 'when' or 'after' in sentences with 'will', look carefully at what you must do! Use the Present Simple tense for the verb after these words. (If you want to be super-correct you say 'after you have fixed', and 'when you have got back', 'after you have finished', but the Present Simple is fine. It's also easier.)
Practice Exercise: make sentences with these words
The situation is two school students talking to each other.
1. if there not be a war, what you do?
2. perhaps I go to University, perhaps I study medicine and become a doctor. I not be a soldier. What about you?
3. if you become a doctor I become your patient.
4. how you know I be a good doctor?
5. I don't know whether you be a good doctor. I hope I not be a patient in any case. I hope I not be ill.
6. You are in good health. Perhaps you never be ill. What you study?
7. I not know. Maybe I be businessman.
8.If you study business you make lots of money.
9. Not as much as a doctor. Maybe I change my mind!
1. If there is a war, what will you do?
2. Perhaps I will go to University, perhaps I will study... I won't be a soldier....
3. If you become a doctor I will be your patient.
4. How do you know I will be a good doctor?
5. I don't know whether you will be a good doctor or not. I hope I won't be a patient in any case. I hope I will not be ill.
6. ....Perhaps you will never be ill. What will you study?
7. I don't know. Maybe I will be a businessman.
8. If you study business you will make lots of money.
9. Not as much as a doctor. Maybe I will change my mind!
See The A - Z of English Grammar and Usage for the rules.