Modal verbs of obligation

1. Modals of obligation

2. How is must used?

• Must can only be used in present.
  

  	Form	Examples
  AFFIRMATIVE	Subject + must + infinitive	I must study every day for my exam. You must be polite with the guests.
  NEGATIVE	Subject + must not / mustn't + infinitive	You mustn't smoke here! You mustn't shout at your brothers.
  INTERROGATIVE	Must + subject + infinitive ...?	Must I do the wahshing-up? Must we go to school?

• Must is used to express that something is essential or necessary from the speaker's point of view. The obligation is based on the speaker's opinion.
  
  We must go home now. It's late.

  I must give my mom a call.
• Must also expresses a strong obligation that comes from a law or regulation, which if we do not respect, it will lead to a penalty or punishment.
  
  You must wear your seat belt.

  You must turn off all electronic devices.
• The negative form mustn't is used to express prohibition.
  
  You mustn't touch the jellyfish.

  You mustn't tell lies.

3. How is have to used?

• Unlike the modal verb must, which only admits obligation in the present tense, have to admits obligation in all verb tenses.
  

  	Structure		Examples
  PRESENT	+	Subject + have / has to + infinitive	You have to do sport every day.
  	-	Subject + don't / doesn't + have to + infinitive	You don't have to do sport every day.
  	?	Do/Does + subject + have to + infinitive ?	Do you have to do sport every day?
  PAST	+	Subject + had to + infinitive	I had to go to the hospital yesterday.
  	-	Subject + didn't + have to + infinitive	I didn't have to go to the hospital yesterday.
  	?	Subject + did + subject + have to + infinitive ?	Did you have to go to the hospital yesterday?
  PRESENT PERFECT	+	Subject + have/has + had to + infinitive	I have had to buy a car.
  	-	Subject + haven't / hasn't + had to + infinitive	I haven't had to buy a car.
  	?	Have / Has + subject + had to + infinitive ?	Have you had to buy a car?
  FUTURE	+	Subject + will + have to + infinitive	You will have to walk the dog tomorrow.
  	-	Subject + won't / have to + infinitive	You won't have to walk the dog tomorrow.
  	?	Will + subject + have to + infinitive ?	Will I have to walk the dog tomorrow?

  In negative and interrogative sentences the verb have to always needs the corresponding auxiliary. Don't use only the verb have.

  You don't have to wash your hair daily.

  You haven't to wash your hair daily.

  Did you have to work on Saturday?

  Had you to work on Saturday?
• Have to is used to express obligation in affirmative sentences. In negative sentences it expresses absence of obligation.
  
  I'll have to wear a tie in my new job.

  You don't have to take the jacket.

4. Differences between have to and must

• In affirmative sentences in the present, both must and have to are interchangeable in many cases. However, the use of have to is preferred when the obligation is external, objective. In contrast, we use must when the obligation comes from the speaker.
  
  I have to pick up my brother from school.

  In this example the boy has the obligation to pick up his brother from school, not because he wants to but because his mother told him to. The obligation comes from his mother (external).

  I must finish the column today.

  In this example we assume that this journalist has decided to finish the article today, not because her boss told her to, but because she feels she needs to do it.
• In negative sentences, unlike mustn't which denotes prohibition, have to expresses absence of obligation or no need to do something.
  
  You didn't have to bring anything.

  In this example, one of the guests has brought a bottle of wine to the party, but it was not necessary to do that.

  We won't have to clean the car, it'll rain.

  In this example, there's no need to wash the car. They could do it if they wanted to, but are not obliged.

Remember!

Must and have to indicate obligation, prohibition or necessity.