Could= possibilities besides conditional uses. could/may/might give same meaning.
It might/may/ could rain tomorrow= it will possibly rain tomorrow. Or maybe it will rain tomorrow.
Maybe(combination of may+be but not an auxiliary and means perhaps)
The leader said there might be a strike next week. I may go to the beach or I may stay home. It might be warmer tomorrow. I may not be able to go with you tonight. John could be at home(I don’t know where sima exactly is) .
Recommendation, advice, obligation.
She should study tonight.
Sima should go on a diet.
You should visit a dentist about this teeth problem.
Should= had better, ought to, be supposed to.
Sima should (had better/ought to/is supposed to) go on a diet.
To indicate complete obligation, stronger than should.( no choice left as in should).
You must endorse a check before you cash in.
She must call her insurance agent today.
The freezer must be kept at -20 degrees.
For logical conclusion , too, must is used.
The leaves are rustling. There must be air flow.
The grass is wet. It must be rainin
She has to call her insurance agent today.
For past obligation, must doesn’t have any words so had to is used.