⚡ Grammar Lesson

Obligation & Necessity (must, have to, should)

🎯 Objective

To help learners master the usage of must, have to, and should to express different degrees of obligation, necessity, and advice in various contexts, understanding when to use each modal appropriately.

📖 Grammar Focus

These modals express different levels of obligation and necessity, from strong requirements to gentle advice. Understanding their differences is crucial for appropriate communication in English.

Modal Strength Usage Example
Must Very Strong (90-100%) Strong obligation, rules, laws You must wear a seatbelt.
Have to Strong (80-90%) External obligation, necessity I have to work tomorrow.
Should Medium (60-70%) Advice, recommendations You should see a doctor.
Ought to Medium (60-70%) Moral obligation, advice We ought to help them.
Need to Strong (70-80%) Necessity, requirement You need to study harder.

📚 Step-by-Step Guide: Mastering Obligation Modals

Understanding Obligation and Necessity

What are Obligation Modals? They express how necessary or important it is to do something, ranging from gentle advice to strict requirements that cannot be ignored.

Key Concept: The choice of modal depends on the strength of the obligation and who is imposing it (internal vs external).

MUST - Strong Obligation and Necessity

1. 🚨 MUST - Formation

Affirmative: Subject + must + base verb

Negative: Subject + must not/mustn't + base verb

Question: Must + subject + base verb?

Examples:

  • You must stop at red lights. (law)
  • Students must not cheat on exams. (prohibition)
  • Must I finish this today? (question)
2. 📋 When to Use MUST

Laws and regulations: You must pay taxes.

Official rules: Passengers must show ID.

Strong personal conviction: I must tell you the truth.

Urgent necessity: We must leave now!

Logical deduction: She must be tired (she's been working all day).

3. 🚫 MUST NOT vs DON'T HAVE TO

Must not (mustn't): Prohibition - not allowed

  • You mustn't smoke here. (It's forbidden)
  • Children mustn't touch the stove. (Dangerous/not allowed)

Don't have to: No obligation - it's optional

  • You don't have to work on Sunday. (It's optional)
  • Students don't have to wear uniforms. (Not required)
4. ⏰ Time Limitations of MUST

Present/Future only: Must is used for present and future

Past reference: Use "had to" for past obligations

  • Present: I must go now.
  • Future: You must submit it tomorrow.
  • Past: I had to work late yesterday. (NOT "I must worked")

HAVE TO - External Obligation

1. 📝 HAVE TO - Formation

Present: I/You/We/They have to / He/She/It has to

Past: had to (all subjects)

Future: will have to

Negative: don't/doesn't have to

Question: Do/Does + subject + have to?

2. 🏢 When to Use HAVE TO

External requirements: I have to work on Saturday. (boss's decision)

Circumstances beyond control: We have to take a detour. (road closed)

General rules: Students have to register before classes.

Practical necessity: I have to buy groceries. (we have no food)

3. ⏳ HAVE TO in Different Tenses

Present: I have to leave early today.

Past: She had to work late yesterday.

Future: We'll have to find another solution.

Present perfect: I've had to change my plans.

Conditional: If it rains, we'll have to cancel.

4. 🆚 MUST vs HAVE TO

MUST: Internal obligation, personal conviction, strong feeling

  • I must call my mother. (I feel I should)
  • You must see this movie! (strong recommendation)

HAVE TO: External obligation, circumstances, other people's rules

  • I have to call my boss. (he asked me to)
  • You have to see a doctor. (it's necessary for health)

SHOULD - Advice and Recommendations

1. 💡 SHOULD - Formation

Affirmative: Subject + should + base verb

Negative: Subject + should not/shouldn't + base verb

Question: Should + subject + base verb?

Examples:

  • You should eat more vegetables. (advice)
  • He shouldn't drive so fast. (advice against)
  • Should I apply for this job? (asking for advice)
2. 🎯 When to Use SHOULD

Giving advice: You should take a vacation.

Making recommendations: We should try that new restaurant.

Expressing expectations: The train should arrive soon.

Moral obligations: People should help each other.

Asking for advice: What should I do?

3. 🤔 SHOULD for Expectations

Expected outcomes: The package should arrive tomorrow.

Probable situations: She should be home by now.

Normal conditions: This medicine should help your headache.

Note: Less certain than "will" but more than "might"

4. 📚 SHOULD HAVE (Past Advice/Regret)

Structure: Should have + past participle

Usage: Advice about past actions, regret

Examples:

  • You should have studied harder. (regret/criticism)
  • I should have called you. (regret about not doing something)
  • She shouldn't have said that. (criticism of past action)

Strength Scale of Obligation

Obligation Strength (Weakest to Strongest):
Could Should Need to Have to Must
  1. Could: You could exercise more. (gentle suggestion)
  2. Should: You should exercise regularly. (advice)
  3. Need to: You need to exercise for your health. (necessity)
  4. Have to: You have to exercise - doctor's orders. (external requirement)
  5. Must: You must exercise or risk serious health problems. (urgent necessity)

Context and Usage Comparison

✅ Appropriate Contexts:
  • Must: Legal requirements, safety rules, strong personal beliefs
  • Have to: Work requirements, practical necessities, external rules
  • Should: Health advice, suggestions, moral guidance
  • Need to: Personal necessities, requirements for goals
❌ Common Mistakes:
  • Using "must" for past: I must went ✗ → I had to go ✓
  • Confusing "mustn't" and "don't have to": Different meanings!
  • Wrong question form: Do you must? ✗ → Must you? ✓
  • Should + -ing: You should studying ✗ → You should study ✓

Other Related Modals

OUGHT TO (Similar to SHOULD):
  • Structure: ought to + base verb
  • Usage: Moral obligation, advice (more formal than "should")
  • Examples: You ought to tell the truth. / We ought to help.
  • Negative: ought not to / oughtn't to (rare)
NEED TO (Necessity):
  • Structure: need to + base verb
  • Usage: Personal necessity, requirement for achieving something
  • Examples: I need to study for the exam. / You need to rest.
  • Different tenses: needed to (past), will need to (future)
BE SUPPOSED TO (Expected Obligation):
  • Structure: am/is/are supposed to + base verb
  • Usage: What is expected, planned, or arranged
  • Examples: I'm supposed to be at work at 9. / The meeting is supposed to start now.
  • Often used when the obligation isn't being followed

🧩 Task: Choose the Right Obligation Modal

Select the most appropriate modal (must, have to, should) for each situation based on the context and strength of obligation.

📝 Choose the best obligation modal for each context:

1. Legal requirement: "All drivers _____ have a valid license to operate a vehicle."

2. Health advice: "You _____ drink more water if you want to stay hydrated."

3. Work obligation: "I _____ attend the meeting tomorrow because my boss scheduled it."

4. Safety prohibition: "You _____ not touch electrical wires with wet hands."

5. Past necessity: "Yesterday, we _____ take a taxi because our car broke down."

1. ✅ Choose the modal for legal requirement:

2. ✅ Choose the modal for health advice:

3. ✅ Choose the modal for work obligation:

4. ✅ Choose the modal for safety prohibition:

5. ✅ Choose the correct form for past necessity:

✅ Answer Key & Explanation

1. ✅ Legal Requirement:

Answer: "All drivers must have a valid license to operate a vehicle."

Explanation: "Must" is perfect for laws and legal requirements. This is a legal obligation that cannot be ignored.

Why not others: "Have to" could work but "must" is stronger for laws. "Should" is too weak for legal requirements.

Context: Legal obligations require the strongest modal to show they are non-negotiable.

2. ✅ Health Advice:

Answer: "You should drink more water if you want to stay hydrated."

Explanation: "Should" is perfect for giving advice and recommendations, especially about health and lifestyle.

Why not others: "Must" is too strong for advice - it would sound like a command. "Have to" implies external pressure.

Context: Health advice allows people to choose, making "should" more appropriate and polite.

3. ✅ Work Obligation:

Answer: "I have to attend the meeting tomorrow because my boss scheduled it."

Explanation: "Have to" is perfect for external obligations imposed by others (boss's requirement).

Why not others: "Must" would suggest internal conviction. "Should" is too weak for a boss's requirement.

Context: External obligations from authority figures call for "have to" rather than personal choice.

4. ✅ Safety Prohibition:

Answer: "You must not touch electrical wires with wet hands."

Explanation: "Must not" (mustn't) expresses strong prohibition, especially for safety reasons where disobedience could be dangerous.

Alternative: "You shouldn't" could work but is weaker. For safety, "must not" is more appropriate.

Context: Safety prohibitions require strong language to prevent dangerous actions.

5. ✅ Past Necessity:

Answer: "Yesterday, we had to take a taxi because our car broke down."

Explanation: "Had to" is the only correct past form for expressing past necessity. "Must" cannot be used for past time.

Rule: For past obligations/necessities, always use "had to" regardless of the present form you would choose.

Context: Past circumstances that forced a particular action require "had to."

🏆 Real-Life Context Examples

Legal and Official Contexts:

Laws and Regulations (MUST):

  • All passengers must fasten their seatbelts during takeoff.
  • Visitors must register at the front desk before entering.
  • You must not use mobile phones during the exam.
  • Drivers must stop at red traffic lights.

Official Requirements (HAVE TO):

  • Students have to submit their assignments by Friday.
  • I have to renew my passport before it expires.
  • Employees have to attend the mandatory training session.
  • You have to be 18 or older to vote.
Health and Medical Contexts:

Medical Advice (SHOULD):

  • You should exercise regularly to stay healthy.
  • People should eat at least five portions of fruit and vegetables daily.
  • You shouldn't ignore persistent headaches.
  • Pregnant women should avoid alcohol and smoking.

Medical Necessity (MUST/HAVE TO):

  • Diabetics must monitor their blood sugar levels. (critical)
  • I have to take this medication three times a day. (doctor's orders)
  • Patients must not eat anything before surgery. (medical requirement)
  • You have to get a prescription for this medicine. (legal requirement)
Workplace Situations:

Company Rules (MUST):

  • All employees must wear safety equipment in the factory.
  • Staff must not share confidential information with outsiders.
  • Everyone must sign in when they arrive at work.

Work Obligations (HAVE TO):

  • I have to finish this report by 5 PM today.
  • She has to travel to London for a business meeting.
  • We have to work overtime to meet the deadline.

Professional Advice (SHOULD):

  • You should update your CV regularly.
  • Managers should give constructive feedback to their team.
  • We should consider hiring more staff for this project.
Educational Settings:

School Rules (MUST):

  • Students must arrive on time for classes.
  • Everyone must turn off their phones during lessons.
  • You must not cheat during examinations.

Academic Requirements (HAVE TO):

  • Students have to complete all assignments to pass the course.
  • I have to write a 2000-word essay for tomorrow.
  • You have to register for classes before the deadline.

Study Advice (SHOULD):

  • You should review your notes after each class.
  • Students should ask questions when they don't understand.
  • You shouldn't wait until the last minute to start studying.
Family and Personal Life:

Strong Personal Beliefs (MUST):

  • I must call my grandmother - it's her birthday.
  • We must tell the children the truth about what happened.
  • I must apologize for what I said yesterday.

Practical Necessities (HAVE TO):

  • I have to pick up the kids from school at 3 PM.
  • We have to buy groceries - the fridge is empty.
  • She has to work late because of the important deadline.

Family Advice (SHOULD):

  • You should spend more time with your family.
  • Children should respect their parents and teachers.
  • We should plan a family vacation this summer.
Travel and Transportation:

Travel Rules (MUST):

  • Passengers must show their boarding passes at the gate.
  • You must not carry liquids over 100ml in hand luggage.
  • Drivers must have valid insurance to drive legally.

Travel Necessities (HAVE TO):

  • I have to be at the airport two hours before my flight.
  • We have to change trains at Central Station.
  • You have to book accommodation in advance during peak season.

Travel Advice (SHOULD):

  • You should check the weather forecast before packing.
  • Travelers should make copies of important documents.
  • You shouldn't leave valuable items unattended in hotel rooms.
Emergency and Safety Situations:

Emergency Procedures (MUST):

  • In case of fire, you must use the stairs, not the elevator.
  • Everyone must evacuate the building immediately when the alarm sounds.
  • You must not return to the building until authorities say it's safe.

Safety Requirements (HAVE TO):

  • Construction workers have to wear hard hats on site.
  • Swimmers have to shower before entering the pool.
  • Visitors have to sign the safety waiver before the tour.

Safety Advice (SHOULD):

  • You should always look both ways before crossing the street.
  • Children should wear helmets when riding bicycles.
  • People shouldn't walk alone in dark areas at night.
Environmental and Social Responsibility:

Environmental Obligations (MUST):

  • Companies must follow environmental protection laws.
  • We must not pollute rivers and oceans with chemicals.
  • Everyone must take responsibility for protecting the planet.

Social Expectations (SHOULD):

  • People should recycle paper, plastic, and glass.
  • We should use public transport to reduce air pollution.
  • Everyone should try to conserve water and electricity.
  • Citizens shouldn't litter in public spaces.
Technology and Digital Life:

Digital Security (MUST):

  • Users must create strong passwords for their accounts.
  • You must not share personal information with strangers online.
  • Companies must protect customer data according to privacy laws.

Platform Requirements (HAVE TO):

  • Users have to agree to terms and conditions before signing up.
  • You have to verify your email address to activate the account.
  • Students have to use the school's learning management system.

Digital Etiquette (SHOULD):

  • You should think carefully before posting on social media.
  • People should respect others' privacy online.
  • You shouldn't believe everything you read on the internet.
Quick Reference: Choosing the Right Modal
Situation Best Modal Example
Laws and legal requirements Must You must pay taxes
Boss's instructions Have to I have to work late
Health and lifestyle advice Should You should exercise
Safety prohibitions Must not You mustn't smoke here
Past obligations Had to I had to leave early
Personal strong feelings Must I must tell you this
External circumstances Have to I have to take a detour
Recommendations Should You should try this
Memory Tips for Mastering Obligation Modals:
  • MUST = Strong internal feeling or official rule (laws, safety)
  • HAVE TO = External pressure or practical necessity (boss, circumstances)
  • SHOULD = Advice and recommendations (you have a choice)
  • MUST NOT = Prohibited, forbidden, not allowed
  • DON'T HAVE TO = Optional, not necessary, your choice
  • Past obligations = Always use "had to" (never "must" for past)