Obligation and no obligation

In the presentObligationI must go home now. (=It’s my decision). (The obligation comes from the speaker. It isn’t a law or a rule)

In Colombia if you are 18, you have to carry your cédula. (=You have no choice). (The obligation comes from somebody else. It’s a law or rule that can’t be changed).

No obligationYou can do something if you want to but it’s not compulsory.

You don’t have to dress up for the party. Wear whatever you feel comfortable in. You don’t have to wear a tie in our office. You can wear a tie if you want to but it’s OK if you don’t.

Permission and prohibition

In the present

They can / are allowed to voteWe can’t / aren’t allowed to / musn’t speak.

Obligation, permission and prohibition

In the pastObligationI had to go home. No obligationI didn’t have to go home.PermissionThey could/were allowed to vote.

ProhibitionWe couldn’t / weren’t allow to speak.

Suggestion (When it is the correct thing to do)

Affirmative

You should try this ice cream – it’s delicious.You ought to be more careful – You’re driving too fast my dear.You are a disorganized person – You ought to make your bed.

Must and have to are stronger than should or ought to.

Rules for Politeness in Different Countries• http://www.ba-bamail.com/content.aspx?emailid=17141