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Modal Verbs of Obligation and Necessity

1. Use of Modal Verbs of Obligation and Necessity

Modal Verbs can be used to talk about obligation, freedom and necessity to do something. We use the following modal verbs:

MustHave toNeed toDon’t have toNeedn’t, Don’t Need to infinitive

2. Talking about Obligation and Necessity in the present

2.1 Obligation

Have to and must are both used to express obligation. There is a slight difference between the way they are used.

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.

  • In Britain you have to wear a seat belt when you drive

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

  • I must finish this before I go home

Sometimes if the speaker agrees with an external rule they will use must

  • In Britain you must wear a seat belt when you drive.

Need to have to and need to are often used in the same context, but many times, need to is used to express something that is less urgent, something in which you have a choice.

  • I have to work late tonight
  • I need to work late tonight

2.2 Forbidden

If something is forbidden must not / mustn’t + base form of the verb

  • You must not drive drink and drive.
  • You mustn’t leave medicines where children can find them.

2.3 No obligation

We use don’t have to to show that there is no obligation. You can do something if you want to but it’s not compulsory. 

don’t /doesn’t /didn’t + have to + base form of the verb

  • You don’t have to wear a tie in our office. You can wear a tie if you want to but it’s OK if you don’t.

needn’t + base form of the verb

  • You needn’t worry about me. I’ll be fine.

3.Talking about Obligation and Necessity in the past

There is no past form of must so we use had to

had to / needed to + base form of the verb

  1. had to work late last night.
  2. needed to drink a few cups of coffee in order to stay awake.

Note: have to and need to are often used in the same context, but many times, need to is used to express something that is less urgent, something in which you have a choice.

3.Talking about Obligation and Necessity in the future

There is no future form of must so we use have to

will have to / will need to + infinitive

  • will have to work late last night.
  • will need to drink a few cups of coffee in order to stay awake.