什麼情況用should/ought to/must?

If your father said to you " You must finish your homework by 9pm ." last
night and now it is today and you want to say what your father said to you
last night to a friend of yours, you would have to change" must " to
" had to ".
Siu Ling,( your friend) my father said I had to finish my homework by 9pm
the other night.( Also, you have to change " last night" to " the other night".
This is what we call " indirect speech" in English grammar.

Hope it helps you .
Regards

should/ought to=應該，但不一定要做
must=一定，有強制的意思