Modal: Must / Mustn’t – Expressing obligation/prohibition

1. Introduction to Modal Verbs: “Must” and “Mustn’t”

Modal verbs are special auxiliary verbs that help express necessity, obligation, or prohibition. In this lesson, we focus on “must” and “mustn’t”. These modals are used to indicate strong obligations or personal convictions as well as to express prohibitions—what is not allowed.

While “must” often conveys an internal, personal need or a logical deduction, “mustn’t” tells us that something is forbidden. Let’s explore each in detail with plenty of examples.

2. Understanding “Must”

A. “Must” for Strong Obligation

“Must” is used when there is a strong necessity or duty that is often driven by personal conviction or internal motivation. It conveys that something is essential or required.

Examples:

• 1. I must complete my project by Friday.
• 2. You must wear a seatbelt at all times.
• 3. Students must arrive at school on time.
• 4. I must study every day to improve my grades.
• 5. You must follow the instructions carefully.
• 6. She must call her mother every evening.
• 7. We must submit our assignments before the deadline.
• 8. He must take his medicine as prescribed.
• 9. You must lock the door when you leave.
• 10. They must stop at the red light.

B. “Must” for Deduction or Certainty

“Must” is also used to draw conclusions based on evidence or logical reasoning. It shows that something is very likely true.

Examples:

• 1. The lights are on; they must be home.
• 2. You are wet; you must have been caught in the rain.
• 3. The ground is wet; it must have rained last night.
• 4. He is carrying a heavy book; he must be a student.
• 5. The clock is ticking; it must be time to leave.
• 6. That actor is very talented; he must have practiced a lot.
• 7. The music is loud; they must be having a party.
• 8. The car isn’t moving; it must be broken down.
• 9. The trees are bare; it must be winter.
• 10. He is smiling; he must be happy.

3. Understanding “Mustn’t”

“Mustn’t” (short for “must not”) is used to indicate prohibition. It tells someone that an action is not allowed, either due to rules, laws, or strong advice.

Usage for Prohibition

Examples:

• 1. You mustn’t smoke in this building.
• 2. Students mustn’t use mobile phones during exams.
• 3. You mustn’t drive without a license.
• 4. You mustn’t eat in the library.
• 5. You mustn’t leave the door unlocked.
• 6. You mustn’t enter the restricted area.
• 7. You mustn’t use your phone during the meeting.
• 8. You mustn’t speak loudly in the theater.
• 9. People mustn’t smoke near the entrance.
• 10. You mustn’t litter in the park.

4. Distinguishing “Must” from “Have to”

Both “must” and “have to” express obligation, but they are used differently:

• “Must” expresses an internal, personal obligation or conviction.
• “Have to” expresses an external obligation imposed by rules, laws, or circumstances.

Examples:

• 1. I must stop eating junk food. (Personal choice)
• 2. I have to take my medicine as the doctor advised. (External requirement)
• 3. I must study hard because I want to succeed. (Internal motivation)
• 4. I have to attend the meeting because it is mandatory. (Imposed by work)
• 5. I must repair my car because I value it. (Personal care)
• 6. I have to repair my car because it broke down. (External circumstance)
• 7. I must read more to improve my knowledge. (Personal goal)
• 8. I have to read this book for my class. (Assignment requirement)
• 9. I must practice to become a better artist. (Personal ambition)
• 10. I have to wake up early because my work starts at 7. (External schedule)

5. Distinguishing “Mustn’t” from “Don’t Have to”

It is important not to confuse “mustn’t” with “don’t have to”.
“Mustn’t” is used to indicate that something is forbidden or not allowed, whereas “don’t have to” means there is no necessity or obligation to do something, but it is allowed.

Examples of “Mustn’t”:

• 1. You mustn’t smoke in this building.
• 2. You mustn’t use your phone during the exam.
• 3. You mustn’t leave the door open.
• 4. You mustn’t speak during the movie.
• 5. You mustn’t cheat on the test.
• 6. You mustn’t share your password.
• 7. You mustn’t eat in the library.
• 8. You mustn’t park in the no-parking zone.
• 9. You mustn’t interrupt the speaker.
• 10. You mustn’t litter in the park.

Examples of “Don’t Have to”:

• 1. You don’t have to wake up early on weekends.
• 2. I don’t have to clean the house today.
• 3. You don’t have to study if you already know the material.
• 4. They don’t have to cook dinner tonight.
• 5. I don’t have to attend the meeting if I’m busy.
• 6. You don’t have to drive if the bus is available.
• 7. I don’t have to play video games if I have work to do.
• 8. You don’t have to call me if you’re busy.
• 9. We don’t have to move if the apartment is comfortable.
• 10. They don’t have to watch that movie if they don’t want to.

6. Forming Sentences with “Must” and “Mustn’t”

The basic structure for using “must” and “mustn’t” is very simple:

Structure: Subject + must/mustn’t + base verb

Examples:

• 1. She must finish her homework.
• 2. They mustn’t park here.
• 3. I must attend the lecture.
• 4. You mustn’t leave the door unlocked.
• 5. He must practice every day.
• 6. We mustn’t waste food.
• 7. I must submit the report by tomorrow.
• 8. You mustn’t interrupt during the meeting.
• 9. She must prepare for her performance.
• 10. They mustn’t litter in the park.

7. Common Mistakes and Clarifications

• Do not use “must” for past obligations; use “had to” instead.
• Avoid confusing “mustn’t” (prohibition) with “don’t have to” (lack of necessity).
• Overusing “must” in informal contexts—sometimes “have to” is more natural.

Remember: “must” expresses strong internal obligation or logical deduction, whereas “have to” is generally used for external requirements. “Mustn’t” strictly means that something is forbidden.