2.1.- Modal verbs for obligation or necessity.

• Must is only for present or future situations and it is used for strong recommendation from the speaker (eg written rules).
  • All students must wear uniforms in this school.
  • Doctor: "You must stay in bed for 4 days".

  It is also used when the speaker thinks something is necessary, because it is his own feelings.

  • I must write to Kate. She is my best friend and I haven't heard of her for one month.
• Have to as a substitute to must: It has all forms:
  • Present: have/has to
  • Past: had to.
  • Present perfect : have had to,
  • Future: will have to
  • Infinitive: to have to
    1. She might not have to go to work tomorrow.
    2. As he didn't the test yesterday, he will have to do it this evening.

  It changes the third person into "has to".

  • She has to play basket today.

  It uses auxiliaries for questions and negatives:

  • Do I have to do homework now, Mum?
  • Why did you have to go to hospital?
  • You don't have to wear a uniform in this school.

What is the main difference between MUST and HAVE TO?

Have to is used when the speaker does not express his feelings, he is just giving facts. The obligation comes from the circumstances or external rules. It is used in general obligations.

• We had to drive on the left in Britain last August.
• My boss has had to leave in an emergency.

Must is used when the obligation comes from the speaker, that is, personal obligations and also more specific ones (i.e. on one occasion)

• You must study this lesson for tomorrow.

Compare:

• I must study this lesson for tomorrow. (It's my decision).
• I have to study this lesson for tomorrow. (My teacher told me).

You can also use must or have to for strong recommendations.

• You must/have to see that film. It's fantastic.