To talk about obligation, you can use different modals, such as : Must, Have to and Need to.

• Must :  used for obligation, especially for a rule or a law

– Affirmative form : Subject + must + infinitive verb (eg. You must reply.)

– Negative form : Subject + must + not + infinitive verb (eg. You mustn’t reply).

– Question form : X —> We cannot use “must” in a question.

• Have to / Need to : used for necessity

– Affirmative form : Subject + have (has) to / need(s) to + infinitive verb (eg. You have to come. You need to come)

– Negative form : Subject + do + not + have (has) to / need(s) to + infinitive verb (You don’t have to come. You don’t need to come)

– Question form : Do + Subject + have (has) to / need(s) to + infinitive verb (Do you have to come? Do you need to come ?)