Lesson

Obligation and necessity

Express what must be done

≈ 15 min 8 block(s)

Text

What are expressions of obligation and necessity?

Expressions of obligation and necessity are phrases we use to say that something is required, essential, or expected. They vary in strength and formality — from strong, mandatory language to softer, polite requests.

  • Indicate rules, duties, or requirements (e.g., must, are required to).
  • Express personal obligation or internal necessity (e.g., need to).
  • Offer advice or weak obligation (e.g., should, ought to).
  • Choose expressions according to tone: legal/formal vs. conversational/polite.

Table

Common expressions of obligation (strong → weak)

Expression Meaning / Strength Example
must Very strong obligation; speaker-centered You must submit the report by 5 PM.
have to / has to External obligation (rules, requirements) Employees have to complete compliance training.
are required to / is required to Formal, official requirement Visitors are required to sign in at reception.
need to Necessity; can be personal or pragmatic I need to finish the budget before the meeting.
ought to Moral advice; softer than must/should You ought to inform the team about the delay.
should Recommendation or expectation You should check the figures twice.
it's necessary to / it's required that Impersonal formal necessity It's necessary to provide ID for access.
have got to Informal strong obligation I've got to catch the 8:00 train.

Tip

How to choose the right expression

Choose based on source of obligation and tone:

  • Use must when the speaker asserts a strong, internal obligation or rule.
  • Use have to for obligations coming from external rules or regulations.
  • Use need to for practical necessity or personal requirement.
  • Use should / ought to for advice or expectations (weaker obligation).
  • Use formal structures (are required to, it's necessary to) in official/business contexts.

Consider formality and who enforces the obligation (you vs. policy).

Example

Examples in context

You must wear your ID badge in the building.

All team members have to attend the monthly review.

I need to prepare the presentation before Monday.

You should inform your manager if there is a delay.

Tip

Common mistakes to avoid

Watch out for universal errors learners make with obligation expressions:

  • Confusing must and have to — remember: must is speaker-centered, have to often indicates external rules.
  • Using must in past contexts (wrong: *I musted) — use had to for past obligation.
  • Omitting to after need when using infinitive (wrong: *need go) — correct: need to go.
  • Mixing modals incorrectly (wrong: *must to, *should to) — modals do not take to.
  • Using informal phrases in formal writing (e.g., 'have got to' in an official report).

Check tense and formality before finalizing your sentence.

Quiz

Choose the sentence that indicates an external rule or company policy:

Hint: Think about obligations that come from rules or regulations.

Quiz

Complete: Employees _____ complete the safety training by Friday.

Hint: Think about a formal phrase used in policies or official instructions

Quiz

Which sentence is the most polite request for a business email?

Hint: Consider tone and politeness suitable for professional communication

Key Points

GrammarPoint

must

modal verb CEFR B2 //mʌst//

Expresses a strong obligation or certainty from the speaker's perspective.

You must complete the compliance form before starting.

Expression

have to

modal-like phrase CEFR B1 //hæv tə/ or /həv tə//

Indicates obligation due to external rules, circumstances, or requirements.

Staff have to wear ID badges at all times.

Expression

need to

expression / semi-modal CEFR B1 //niːd tuː//

Expresses necessity or requirement, often practical or personal.

I need to send the updated files before lunch.

Expression

are required to

passive phrase CEFR B2 //ər rɪˈkwaɪərd tuː//

A formal passive construction indicating an official requirement.

Visitors are required to present identification at the entrance.

GrammarPoint

should

modal verb CEFR B1 //ʃʊd/ or /ʃəd//

Used for advice, recommendations, or expectations; weaker than must/have to.

You should review the contract with legal before signing.

Expression

ought to

modal phrase CEFR B2 //ɔːt tuː/ or /ˈɔːt tuː//

Expresses moral recommendation or expected behavior; somewhat formal.

You ought to notify the client about the delay immediately.