Modals of Obligation, Prohibition and Advice: Must, Have to, Should, Ought to — Grammar Rules and BAC-Style Exercises — Second Year Secondary — English — Algerian Curriculum

Modals of Obligation, Prohibition and Advice

Learning Objectives:

• Understand the difference between modals of obligation, prohibition, and advice
• Use “must” and “have to” correctly to express obligation
• Use “mustn't” to express prohibition
• Use “should” and “ought to” to give advice
• Apply these modals in BAC-style exercises

1. Introduction to Modals:

Modal verbs are auxiliary verbs that express necessity, obligation, permission, ability, etc. They are followed by the base form of the verb (without “to”, except for “ought to”).

2. Modals of Obligation:

Modal	Usage	Example
Must	Strong obligation (personal feeling)	I must study for the exam.
Have to	External obligation (rules/laws)	Students have to wear uniforms.
Has to	External obligation (3rd person singular)	She has to arrive on time.

Note: “Must” has no past tense. For past obligation, use “had to”.

• Present: I must go / I have to go
• Past: I had to go (NOT: I musted go)

3. Modals of Prohibition:

Modal	Usage	Example
Mustn't	Strong prohibition (forbidden)	You mustn't cheat in exams.
Don't have to / Doesn't have to	Lack of obligation (not necessary)	You don't have to come if you're tired.

Important difference:

• Mustn't = It is forbidden (prohibition)
• Don't have to = It is not necessary (absence of obligation)

4. Modals of Advice:

Modal	Usage	Example
Should	Giving advice or recommendation	You should drink more water.
Shouldn't	Negative advice	You shouldn't stay up late.
Ought to	Strong advice (formal)	You ought to respect your parents.
Ought not to	Negative strong advice	You ought not to lie.

5. Summary Table:

Modal	Meaning	Strength
Must	Obligation	★★★★★
Have to	External obligation	★★★★★
Mustn't	Prohibition	★★★★★
Don't have to	No obligation	★★☆☆☆
Should	Advice	★★★☆☆
Ought to	Strong advice	★★★★☆

6. BAC-Style Exercises:

Exercise 1: Choose the correct modal to complete each sentence:

1. You _______ (must / should) stop smoking. It's bad for your health.
2. Students _______ (mustn't / don't have to) run in the corridors. It's against the rules.
3. I _______ (have to / should) finish this project by Friday. The boss needs it.
4. You _______ (don't have to / mustn't) bring your dictionary. I'll lend you mine.

Answers:

1. should (advice)
2. mustn't (prohibition)
3. have to (external obligation)
4. don't have to (no obligation)

Exercise 2: Rewrite the sentences using the modal in brackets:

1. It is necessary for all citizens to respect the law. (must)
2. It is forbidden to use mobile phones in class. (mustn't)
3. I advise you to see a doctor. (ought to)
4. It is not necessary for you to pay now. (don't have to)

Answers:

1. All citizens must respect the law.
2. You mustn't use mobile phones in class.
3. You ought to see a doctor.
4. You don't have to pay now.

Exercise 3 (BAC-style): Complete the paragraph with the correct modals:

“As a student, you must attend all classes regularly. You mustn't be late because the teacher locks the door. You have to submit your homework on time. However, you don't have to buy the expensive textbook; the library has copies.”

7. Summary:

• Must = strong personal obligation
• Have to = external obligation (rules)
• Mustn't = prohibition (forbidden)
• Don't have to = lack of obligation (not necessary)
• Should = advice / recommendation
• Ought to = strong advice (formal)

📚 Related Lessons:

• Relative Pronouns: Who, Which, That, Whose
• Conditional Sentences Types 1, 2, 3
• The Passive Voice — Formation, Rules and BAC Exercises