Moral duties/obligations
We must always speak the truth.
We must not tell a lie.
You must always help the poor.
Probability of action
When you are having 99% surety of any action to happen
When I invited him to attend my birthday party, he got very happy.
He must be excited to join it.
Necessity
We must awake early in the morning to keep ourselves fit and fine.
We must always keep ourselves up to date.
Must we use this word?
Must we really leave for Delhi right now?
Must we really use the marks while moving out of the home?
Prohibition
We must not cross the road when the light turns red.
Fixed Expected /Determination
She thinks like me she must be my friend.
The person taking a round of the school must be the Principal.