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

🔑 Lesson 1: Modals of Obligation & Necessity

Welcome to our new module on modal verbs¹! These are special verbs that express ideas like certainty, possibility, or, for today's lesson, obligation² and necessity³. We will learn how to use must, have to, mustn't, and don't have to to talk about rules and requirements.

Strong Obligation: "must" and "have to"

Both "must" and "have to" express a strong obligation, meaning something is required. In many positive sentences, their meanings are very similar.

• must: Often shows an internal⁴ obligation (the speaker feels it is important) or a formal, written rule.
  Example: "I must study hard for my exam." (This is my personal feeling.)
• have to: Often shows an external⁵ obligation (a rule from outside, like a law).
  Example: "In Cambodia, you have to drive on the right side of the road." (This is the law.)

The Big Difference: "mustn't" vs. "don't have to"

This is the most important part of the lesson! These two phrases look similar but have completely different meanings.

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."

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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 Quiz: 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.
   Answer: mustn't (This is a prohibition, a rule against something.)
2. To enter the country, you _______ have a valid passport.
   Answer: have to / must (This is a strong obligation.)
3. The English class is optional. You _______ come if you're busy.
   Answer: don't have to (It is not necessary; you have a choice.)
4. I _______ remember to call my mother for her birthday tomorrow.
   Answer: 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.
   Answer: have to (This is an external rule/law.)

📝 Homework: 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.
   (Example Answer: You mustn't speak loudly in the library.)
   _________________________________________
2. Situation: Your friend offers to pay for your coffee, but you have money.
   (Example Answer: Thank you, but you don't have to pay for me.)
   _________________________________________
3. Situation: A sign at the entrance to the Battambang Bat Caves.
   (Example Answer: You must be careful on the stairs.)
   _________________________________________

Vocabulary Glossary

1. Modal Verb: (Noun) - កិរិយាស័ព្ទគន្លឹះ (kĕ'rĭ'ya'sâp kôn'lœ̆h) - A special verb (like must, can, should) that changes the meaning of the main verb. ↩
2. Obligation: (Noun) - កាតព្វកិច្ច (ka'tâp'vĕ'kĕch) - Something that you must do because of a rule, law, or promise. ↩
3. Necessity: (Noun) - ការចាំបាច់ (kaa cham'bach) - The fact of being required or necessary. ↩
4. Internal / External: (Adjectives) - ខាងក្នុង / ខាងក្រៅ (kʰaŋ knong / kʰaŋ kraeuy) - Inside / Outside. Internal obligation is a feeling from inside yourself; external is a rule from outside. ↩
5. Prohibition: (Noun) - បម្រាម (bâm'ram) - A rule that says something is not allowed. ↩