Grammar: B1 - 🔑 Modals & Related Structures: Certainty, Possibility, Advice, Obligation - Lesson 1: Modals of Obligation & Necessity (must, have to, don't have to, mustn't

Grammar: Modals of Obligation & Necessity

B1 Lesson 1: The Basics

What you will learn: By the end of this lesson, you will be able to use must, have to, mustn't, and don't have to to talk correctly about rules and requirements.

Strong Obligation vs. No Obligation

Modal verbs are special verbs that express ideas like certainty, possibility, or, for today's lesson, obligation and necessity. We will learn the important difference between things that are required and things that are not.

"must" / "have to"

These both express a strong obligation. Must is often for personal feelings or formal rules. Have to is often for external rules and laws.

Example: "I must remember to call my mom." (Personal)
"You have to wear a helmet on a moto." (Law)

"mustn't" / "don't have to"

These have very different meanings! Mustn't is a prohibition (Don't do it!). Don't have to means something is not necessary (you have a choice).

The Grammar Rule 📖

The Big Difference: Prohibition vs. No Necessity

mustn't = Prohibition (It is forbidden)

Use "mustn't" to say something is against the rules or a very bad idea. It means "Do NOT do this!"

"You mustn't touch the ancient carvings at the temples."
"This is a secret. You mustn't tell anyone."

don't have to = No Obligation (It is not necessary)

Use "don't have to" when there is no requirement. You have a choice. You can do it if you want, but it's not necessary.

"You don't have to pay to enter the pagoda, but a small donation is appreciated."
"My friend offered me a ride, so I don't have to walk."

Practice Your Grammar 🎯

Exercise: Choose the Correct Modal

Choose the best option to complete each sentence.

1. It's a rule at the museum. You _______ take photos with a flash.
   → mustn't (This is a prohibition, a rule against something.)
2. To enter the country, you _______ have a valid passport.
   → have to / must (This is a strong obligation.)
3. The English class is optional. You _______ come if you're busy.
   → don't have to (It is not necessary; you have a choice.)
4. I _______ remember to call my mother for her birthday tomorrow.
   → must (This is a strong internal feeling of obligation.)
5. When you ride a moto in Cambodia, you ______ wear a helmet. It's the law.
   → have to (This is an external rule/law.)

Your Grammar Mission ⭐

What's the Rule?

Write a sentence for each situation using one of the modals we learned today.

1. Situation: In a library, regarding noise.
2. Situation: Your friend offers to pay for your coffee, but you have money.
3. Situation: A sign at the entrance to the Battambang Bat Caves.

Key Vocabulary

• Modal Verb (Noun) | កិរិយាស័ព្ទគន្លឹះ
  A special verb (like must, can, should) that changes the meaning of the main verb.
• Obligation (Noun) | កាតព្វកិច្ច
  Something that you must do because of a rule, law, or promise.
• Necessity (Noun) | ការចាំបាច់
  The fact of being required or necessary.
• Internal / External (Adjectives) | ខាងក្នុង / ខាងក្រៅ
  Inside / Outside. Internal obligation is a feeling from inside yourself; external is a rule from outside.
• Prohibition (Noun) | បម្រាម
  A rule that says something is not allowed.