Grammar: Obligation

Must

Shows us that the obligation comes from the speaker. It isn’t a law or a rule. 

• I must call my dad tonight.
• You must hand in your homework on Tuesday or your mark will be zero.
• You must come and visit us the next time you come to London.

Have to

Shows us that the obligation comes from somebody else. It’s a law or a rule and the speaker can’t change it.

• Do you have to wear a uniform at your school?
• John can’t come because he has to work tomorrow.
• In Britain you have to buy a TV licence every year.

Should

When we want to talk about obligation we are talking about the right thing to do:

• You should to finish your homework before you watch TV.
• You shouldn’t to leave lights on when you leave the room.

Should to are also used to say what is the best thing to do or to give recommendations:

• You should to walk to work.
• The weather in Malta is perfect. You should to come over for a holiday.

Sources:

https://learnenglish.britishcouncil.org/es/grammar-reference/modals-1

http://www.ecenglish.com/learnenglish/lessons/modals-should-ought-obligation-probability