Must express a necessity, a command or a probability.
Ought to and should express moral obligation or a probability.
Verb ought is more emphatic than verb should. Verb should is often used when subject is asking or giving advice. Also should often indicates a recommendation rather than obligation.
Also note that the negative form of verb must is doesn’t express prohibition.
Those verbs express necessity, obligation, command, moral obligation, prohibition, possibility.
Necessity
I must finish my work.
You must wash you t-shirt.
They must tell you the truth
Obligation
I must do it as my parents told me.
Politician must answer on public questions.
You’ve must cross the road here.
Command
You must go home.
You must clean your room.
You must finish your homework.
Moral obligation
I ought to be good soldier.
We ought to finish this project.
You should apologize to your parents.
We should work harder for future of mankind.
Possibility
She must be at work now.
This is expensive TV, It must have good picture.
He have Ferrary, so that car must be in red color.
