could/may/might.
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) .
should.
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.
must
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
have to.
Have to=must.
She has to call her insurance agent today.
For past obligation, must doesn’t have any words so had to is used.
