/Modal Verbs
B1Modal Verbs

Modal Verbs: Obligation

1

What is it?

To express obligation and necessity in English, we use must, have to, need to, and should. "Must" expresses strong personal obligation or rules. "Have to" is used for external obligations (rules set by others). "Should" gives advice or weaker obligation. "Must not" and "don't have to" have very different meanings.

2

How to form it

SubjectPositiveNegativeQuestion
Strong obligation (personal/rule)You must wear a seatbelt. / I must call her.You must not smoke here. (prohibition)Must we submit the form today?
External obligation (set by others)She has to work on Saturdays. / I had to stay late.You don't have to come if you're busy. (no obligation)Do you have to wear a uniform?
Advice / mild obligationYou should apologise.You shouldn't stay up so late.Should I bring anything?
  • • "Must" is used for strong obligations, especially the speaker's own feelings: "I really must call her."
  • • "Have to" is used for external rules or obligations: "We have to wear ID badges at work."
  • • KEY DIFFERENCE: "Mustn't" = prohibition (forbidden); "Don't have to" = no obligation (not necessary).
  • • "Should" is softer than "must" — it's advice, not a strict rule.
  • • "Had to" is the past tense of both "must" and "have to": "I had to stay late yesterday."
  • • "Need to" is similar to "have to" and can be used in most contexts.
3

When to use it

  1. 1

    Strong obligation from the speaker

    I must remember to send that email.

  2. 2

    External obligation (law, rules, other people)

    Drivers have to stop at red lights.

  3. 3

    Prohibition (forbidden)

    You mustn't park on double yellow lines.

  4. 4

    No obligation (not necessary)

    You don't have to finish it today — take your time.

  5. 5

    Advice / recommendation

    You should try the new restaurant.

  6. 6

    Past obligation (any type)

    She had to work last weekend.

4

Common mistakes

You don't have to smoke here. It's forbidden.

You mustn't smoke here. It's forbidden.

"Don't have to" means there is no obligation. For prohibition, use "mustn't".

I must to call her.

I must call her.

"Must" is followed by the bare infinitive — no "to".

He musted work last Saturday.

He had to work last Saturday.

"Must" has no past tense form — use "had to" for past obligation.

5

Quick reference

  • "Must" = strong personal obligation or strict rule.
  • "Have to" = external obligation (rules, laws, others).
  • "Mustn't" = prohibition (you are NOT allowed).
  • "Don't have to" = no obligation (you can, but you don't need to).
  • "Should" = advice or mild obligation.
  • "Had to" = past tense for all obligation modals.
6

Natural conversation example

A new employee is asking a colleague about the office rules.

N

New employee

Do I have to wear a tie every day?
C

Colleague

No, you don't have to — it's not required. But you must wear your ID badge at all times.
N

New employee

And what about working hours? Can I start late sometimes?
C

Colleague

You have to be here by 9. But you don't have to stay past 5 unless there's a deadline.
N

New employee

What mustn't I do? Are there strict rules?
C

Colleague

You mustn't share confidential client information outside the office.
N

New employee

That makes sense. Should I speak to HR about the rest?
C

Colleague

You should — they'll give you the full induction pack.

Practice Exercises

Fill in the blank with the correct modal.

  1. 1.
    You __ wear a seatbelt by law. (legal requirement)
  2. 2.
    She __ work on Sundays — it's part of her contract. (external rule)
  3. 3.
    You __ smoke in here. It's strictly forbidden. (prohibition)
  4. 4.
    You __ come if you don't want to. It's optional. (no obligation)
  5. 5.
    I really __ call my parents — I haven't spoken to them in weeks. (personal obligation)
  6. 6.
    You __ see a doctor about that rash. (advice)
  7. 7.
    She __ stay late last night to finish the report. (past obligation)
  8. 8.
    Students __ submit their essays by Friday. (external rule)
  9. 9.
    You __ eat vegetables every day for good health. (recommendation)
  10. 10.
    I __ bring a pen — they provide everything. (no obligation)
Modal Verbs of AbilityAll TopicsModal Verbs of Probability

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