salam bro
there are some special usages of must,have to ,use to,might,may,shall,will,rare
must = 100%
shall/will = 70%
have to 80%
may might = 40%
rare/seldom = 20%
this percentage shows the power of will or order
i must go means in any condition
i will go means it is necessary
i have to go means something opposite occur if i d not go
i rare go to cenima means in a year i go same is seldom
if u wana say "mujhaiy lahore jana haiy"
so mostly we say
i have to go to lahor
when u say that i must be going to lahore or i have to be going to lahore(usueally we do not use have to be going we use shall be going) so all this means that u r arguing with someoene for some reason and u say no"i must be going to lahore + time that is at that time
i wish u wld have understood what i wrote
wish u alll best of luck
Alllah hafiz