must and have to are the same concerning necessity i must help mum clean the house because she is tired. i have to help mum clean the house because she is tired. in obligation: must is used when this obligation is external; i must do this work, or the boss will be angry. have to is used when the obligation is enternal: i have to do my homework before i go to the club. ought to= should when used to give advice, but should is widely used: you ought to do your homework. you should do your homework. need to expresses necessity to: i need to clean my house before i go out.

http://www.learn-english-today.com/lessons/lesson_contents/should-ought-need.htm http://www.onestopenglish.com/support/ask-the-experts/grammar-questions/grammar-teaching-the-modals-ought-to-should-must-and-have-to/146365.article http://linguapress.com/grammar/modal-obligation.htm