often say I think...will.../I think Kelly will pass the exam.
do not use will for things we have already arranged or decided to do
I'm not working tomorrow.(not I won't work)
I shall=I will and we shall=we will
do not use shall with you/they/he/she/it
use i'll...when you offer something or decide to do someting(short time)
do not use I'll...for someting you decided before
I think I'll.../I don't think I'll...
Shall I/Shall we...?=Do you think this is a good thing to do?Do you think this is a good idea?