Have to
Use: To talk about external rules, laws, or obligations (something is necessary).
Form:
Subject + have/has to + base verb.
- Students have to wear uniforms. ( School's rule)
- People have to wear helmets whenever they ride. ( Country laws or rules)
- She has to do the dishes. ( Her parents tell her to do that.)
Don't have to
Use: no obligation, something optional or not necessary.
Form:
Subject + don't have to + base verb.
- I didn’t have to go to school yesterday. (It was not necessary.)
- You don't have to take of your shoes here. ( It is not required.)
- I don't have to get up early on the weekend. ( It is not important for me.)
Note: “Don't have to” does NOT mean “mustn’t”
Must
Use:
- Often used by the speaker.
Form:
Subject + must + base verb.
- You must stop smoking if you want to stay healthy. ( My advice)
- Students must be on time. ( I think it is necessary.)
- I must finish this work today. ( My determination)
Mustn’t
Use: To say something is not allowed (prohibition).
Form:
Subject + mustn’t + base verb
- You mustn’t smoke here.
- Students mustn’t cheat in exams.
- We mustn’t be late.
⚠️ Important: “Mustn’t” = forbidden
Should
Use: To give advice or say what is a good idea.
Form:
Subject + should + base verb
Examples:
- You should study every day.
- He should eat healthy food.
- We should respect our parents.Shouldn’t
Use:
👉 To give negative advice (something is not a good idea).
Form:
Subject + shouldn’t + verb
- You shouldn’t stay up late.
- She shouldn’t eat too much sugar.
- We shouldn’t be rude.
🌟 Quick Comparison Table
| Modal | Meaning | Example |
|---|---|---|
| have to | necessary / rule | I have to wear a uniform. |
| don't have to | not necessary | I don't have to wear uniform on Saturday. |
| must | strong obligation | You must obey the rules. |
| mustn’t | forbidden | You mustn’t cheat. |
| should | advice | You should exercise. |
| shouldn’t | bad idea | You shouldn’t lie. |
Here is a clear and easy explanation of the difference between “have to” and “must” when we talk about rules 👇
1. Have to (Rules from outside)
We use have to when a rule or obligation comes from another person, law, school, or system.
It means you have no choice because someone else made the rule.
Examples:
Students have to wear a uniform at school.
(School rule)
Drivers have to stop at a red light.
(Traffic law)
Employees have to arrive at work at 8 a.m.
(Company rule)
👉 Have to = external rule
2. Must (Strong rule or speaker’s authority)
We use must when the speaker says the rule or feels it is very important.
It often sounds stronger and more serious.
Examples:
You must be quiet in the library.
(The speaker gives the rule)
You must wear a helmet. It’s dangerous.
(Strong advice / rule)
Visitors must show their ID.
(Official or formal rule)
👉 Must = strong rule from the speaker or authority
3. Simple comparison
4. Important note (modern English)
In spoken English, people often use have to more than must, even for rules.
Must is common in written rules, signs, and formal instructions.
✅ Short summary:
Have to → rule from outside
Must → strong rule or speaker’s authority
If you want, I can also explain this in Khmer or give student exercises 😊
Modal Verbs Practice
Complete the exercises below and check your answers.