Obligation and lack of obligation - MUST, HAVE TO, CAN...

Personal obligations                                       – MUST/NEED TO

I must call my mother

If the speaker has the authority                    – MUST/NEED TO

You must lose some weight

To strongly recommend something               – MUST(HAVE TO)

You must see that film

To speak about rules                                      – HAVE TO/BE SUPPOSED TO

We have to be on time for class

If there is no obligation                                   – DON'T HAVE/NEED TO

You don't have to bring your book

To speak about something that is possible     – CAN/BE ALLOWED TO/

I can leave early today

You are allowed to work from home

To speak about prohibitions you use              – CAN'T/MUSTN'T

You can't smoke here                              NOT BE ALLOWED/SUPPOSED TO

To give advice or express a weak obligation  – SHOULD/OUGHT TO

You should do your homework

You ought to see the doctor