Lesson

Must and can't

Logical deduction

≈ 15 min 8 block(s)

Text

What are 'must' and 'can't'?

The modal verbs must and can't are used to express strong certainty, logical deductions, obligations, and prohibitions. They are short, common, and important for formal and business communication.

  • Must often shows obligation or a strong conclusion (deduction).
  • Can't shows impossibility or strong negative deduction, and with mustn't it shows prohibition.
  • Use must/have to for requirements; use modal perfect forms (must have / can't have) for past deductions.

Table

Uses of 'must' and 'can't'

Use Meaning Example
Obligation (present) Strong requirement Employees must show ID at the entrance.
Prohibition Not allowed You mustn't use your phone in the meeting.
Logical deduction (positive) Strong conclusion about present/future He must be the new manager — he introduced the project.
Logical deduction (negative) Strong conclusion of impossibility She can't be in the office; her laptop is at home.
Past deduction (modal perfect) Conclusion about past events They must have finished the audit already.
Past negative deduction (modal perfect) Strong belief that something did not happen He can't have received the email — he was offline all day.

Tip

Key rule: Form and use

Remember the basic forms and when to use them:

  • Structure: must + base verb (must do), can't + base verb (can't go).
  • Prohibition: mustn't + base verb (mustn't disclose confidential data).
  • Past deductions: must have + past participle / can't have + past participle.

Must and can't express strong certainty — choose 'have to' for external obligations and 'must' for internal obligation or rules.

Example

Examples in context

You must complete the compliance training by Friday.

Employees mustn't share confidential client information.

He must be the new supplier — he signed the contract this morning.

They can't have approved the budget yet; the committee met after hours.

Tip

Common mistakes to avoid

Watch out for these universal errors:

  • Using must for past obligation without the correct form (use had to for past necessity).
  • Confusing mustn't (prohibition) with not necessary — mustn't means 'not allowed'.
  • Using can't for weak possibility; can't expresses strong impossibility, use might/could for possibility.
  • Using two modals together (e.g., must can) — English does not allow double modals.

Check tense and strength of certainty before choosing must, have to, might or can't.

Quiz

Choose the correct sentence:

Hint: Think about which modal expresses prohibition, not impossibility.

Quiz

Complete: The CEO reviewed the figures and _____ have approved the proposal earlier.

Hint: Think about the modal used for a strong logical conclusion in the present.

Quiz

Choose the best option to express impossibility:

Hint: Focus on the modal that shows strong impossibility about the present.

Key Points

GrammarPoint

must

modal verb CEFR B1 //mʌst//

Used to express obligation, necessity, or a strong logical conclusion.

You must submit the invoice by Monday.

GrammarPoint

mustn't

modal verb CEFR B1 //ˈmʌsənt//

Negative form of must; used to express prohibition (not allowed).

You mustn't disclose confidential information.

GrammarPoint

can't

modal verb CEFR B1 //kɑːnt/ or /kænt//

Used to express impossibility or strong negative deduction; with have it forms past deductions (can't have + past participle).

She can't be the author — she left the company last year.

Expression

must have

modal verb + perfect CEFR B2 //mʌst hæv//

Modal perfect used to express a logical conclusion about a past action.

They must have completed the audit before the deadline.

Expression

can't have

modal verb + perfect CEFR B2 //kænt hæv/ or /kɑːnt hæv//

Modal perfect used to express a strong belief that something in the past was impossible.

He can't have missed the meeting; he was on the security camera.

GrammarPoint

have to

phrase (modal-like) CEFR B1 //hæv tuː/ or /ˈhæv tə//

Expresses external obligation or requirement (often imposed by rules or others).

I have to attend the compliance workshop this afternoon.