1. Predictions about the future (based on opinions and experience).
One day people will  travel to Mars.
2. Instant (on-the-spot) decisions (we decide to do something at the moment of speaking).
Look at that wonderful bag! I will  buy it immediately!
3. Actions that will inevitably happen.
(Today is Monday.) It will  be Tuesday tomorrow.
4. Speaking about
a) offers
Your bag is heavy. I will  help you.
b) requests
Will you shut the door, please?

c) promises
I won’t tell anyone.

d) warnings 
Be careful with the knife or you will  cut yourself.
e) threats
Stop doing this or I will  punish you.
5. With the words/expressions:

*Definitely  - Computers will definitely be cheaper.
*(I’m) certain that
*It’s certain that
*It’s quite certain that
*probably - Cars will probably be cheaper.
*perhaps - Perhaps air travel will  be cheaper.
*It’s possible that
*(I) think - I think the exam will  be difficult.
*(I) suppose
*(I) know
*(I) expect
*(I) believe
*(I) hope
*(I’m) sure - I’m sure you’ll  pass the exam.
*(I’m) afraid
*I wonder - I wonder what will  happen.