A1 Advanced Modals 4 min read

Must vs.

Use `must` for absolute requirements and rules, but never add `to` or change its form for different subjects.

The Rule in 30 Seconds

  • Use `must` for 100% necessary rules, laws, and very strong personal obligations.
  • Never use `to` after `must`. Use the base verb only (e.g., `must go`).
  • The form never changes. No `s` for he/she/it and no past tense forms.
  • `Mustn't` means forbidden. `Don't have to` means it is optional or not required.

Quick Reference

Function Structure Example Sentence How it Feels
Strong Rule Subject + must + verb You must stop at the red light. Mandatory
Prohibition Subject + mustn't + verb You mustn't park here. Forbidden
Strong Advice Subject + must + verb You must see that new movie! Enthusiastic
Logic/Guess Subject + must + be It's 5:00. He must be at home. Certain
Internal Need Subject + must + verb I must remember to buy milk. Personal
Formal Law Subject + must + verb All visitors must sign in. Official

Key Examples

3 of 8
1

You must wear a helmet when riding a bike.

Debes usar un casco al andar en bicicleta.

2

I must call my grandmother today; it is her birthday.

Debo llamar a mi abuela hoy; es su cumpleaños.

3

The ground is wet. It must be raining outside.

El suelo está mojado. Debe estar lloviendo afuera.

💡

The 'No-To' Rule

Think of `must` and the next verb as best friends who hate the word `to`. They never let `to` stand between them. Always say `must go`, never `must to go`.

⚠️

Danger: Mustn't vs Don't Have To

If you tell your boss 'You mustn't come to my meeting,' you are telling them they are banned! If you mean it's not necessary, say 'You don't have to come.'

The Rule in 30 Seconds

  • Use `must` for 100% necessary rules, laws, and very strong personal obligations.
  • Never use `to` after `must`. Use the base verb only (e.g., `must go`).
  • The form never changes. No `s` for he/she/it and no past tense forms.
  • `Mustn't` means forbidden. `Don't have to` means it is optional or not required.

Overview

Welcome to the world of must. Think of must as the boss of English words. It is strong. It is serious. It does not like to argue. When you use must, you are talking about things that are 100% necessary. There is no middle ground here. It is like a grammar traffic light that is always stuck on red or green. You use it for rules, laws, and very strong feelings. Even native speakers get a bit nervous using it sometimes! But don't worry. We will make it simple for you. By the end of this, you will use must like a pro.

How This Grammar Works

Most English verbs are like high-maintenance pets. They need different endings for different people. Not must. It is the easiest roommate you will ever have. It never changes. You don't add an s for he or she. You don't add ed for the past. It stays exactly the same every single time. It also refuses to work with the word to. You just put the main action right after it. It is clean, fast, and very direct. It’s like a straight line from the subject to the action.

Formation Pattern

  1. 1Creating a sentence with must is as easy as making toast. Follow these three steps:
  2. 2Start with your Subject (I, You, The cat).
  3. 3Add must (or mustn't for negatives).
  4. 4Add the Base Verb (the simple form like eat, go, or stop).
  5. 5For example: You + must + wear + a seatbelt.
  6. 6For negatives: You + mustn't + smoke + here.
  7. 7Note: We do NOT say must to go. That is a grammar crime! Just say must go.

When To Use It

There are four main times to call the boss:

  • Official Rules and Laws: Think of signs at the airport. Passengers must show their passports. It isn't a suggestion. If you don't do it, you don't fly!
  • Internal Necessity: This is when YOU feel it is important. I must call my mom. Nobody is forcing you, but your heart says it's a rule.
  • Strong Advice: When you really want a friend to do something. You must try this pizza! It is amazing! It’s like a friendly command.
  • Logical Certainty: This is the 'detective' use. He has three cars. He must be rich. You are 99% sure it is true.

When Not To Use It

Even a boss has limits. Do not use must in these situations:

  • The Past: Must lives only in the present and future. For yesterday, use had to. I had to work yesterday, not I musted work.
  • Polite Questions: Asking Must you make that noise? sounds very annoyed. It’s like asking, 'Do you really have to be that annoying?' Use Do I have to...? instead.
  • Optional Things: If something is just a good idea but not a rule, use should. Must is for when the 'grammar police' might show up.

Common Mistakes

Everyone makes mistakes. Even your English teacher probably did once! Here are the big ones to avoid:

  • The "To" Trap: People often say I must to study. This is wrong. Keep must and the verb close together with no to between them.
  • The Third Person "S": Never say She musts go. Must is a rebel; it hates the letter s at the end.
  • Mustn't vs. Don't Have To: This is the biggest one. Mustn't means 'Do not do this! It is forbidden!' Don't have to means 'You can do it if you want, but you don't need to'. If you tell a friend You mustn't come to the party, you are uninviting them! If you say You don't have to come, you are being a nice friend.

Contrast With Similar Patterns

Let's compare must with its cousin, have to. They are very similar, but they have different vibes.

  • Must is often about internal rules. I decided it. I must exercise more.
  • Have to is often about external rules. Someone else decided it. I have to wear a uniform at work.
  • In the negative, they are totally different. You mustn't eat that (It is poison!). You don't have to eat that (If you aren't hungry, it's okay).

Quick FAQ

Q. Is must more formal than have to?

A. Yes, you will see must on signs and in books more often than in casual chat.

Q. Can I use must for the future?

A. Yes! I must finish this tomorrow works perfectly.

Q. Is mustn't common in American English?

A. Not really. Americans usually say can't or not allowed to. Mustn't sounds very British and fancy to them.

Reference Table

Function Structure Example Sentence How it Feels
Strong Rule Subject + must + verb You must stop at the red light. Mandatory
Prohibition Subject + mustn't + verb You mustn't park here. Forbidden
Strong Advice Subject + must + verb You must see that new movie! Enthusiastic
Logic/Guess Subject + must + be It's 5:00. He must be at home. Certain
Internal Need Subject + must + verb I must remember to buy milk. Personal
Formal Law Subject + must + verb All visitors must sign in. Official
💡

The 'No-To' Rule

Think of `must` and the next verb as best friends who hate the word `to`. They never let `to` stand between them. Always say `must go`, never `must to go`.

⚠️

Danger: Mustn't vs Don't Have To

If you tell your boss 'You mustn't come to my meeting,' you are telling them they are banned! If you mean it's not necessary, say 'You don't have to come.'

🎯

Sound Like a Native

In conversation, use `must` for things you are excited about, like `You must see this!`. For boring rules, native speakers usually use `have to`.

💬

British vs American

British speakers love `mustn't`. Americans think it sounds like a character from a Harry Potter book. In the US, people usually say `can't` or `shouldn't` instead.

Exemplos

8
#1 Basic Rule

You must wear a helmet when riding a bike.

Focus: must wear

Debes usar un casco al andar en bicicleta.

This is a safety rule that is mandatory.

#2 Internal Necessity

I must call my grandmother today; it is her birthday.

Focus: must call

Debo llamar a mi abuela hoy; es su cumpleaños.

The speaker feels a strong personal obligation.

#3 Edge Case (Deduction)

The ground is wet. It must be raining outside.

Focus: must be

El suelo está mojado. Debe estar lloviendo afuera.

Using must to make a logical guess based on evidence.

#4 Formal Instruction

Candidates must arrive fifteen minutes before the exam.

Focus: must arrive

Los candidatos deben llegar quince minutos antes del examen.

Commonly found in official documents or exam rules.

#5 Mistake Corrected

✗ She musts to go → ✓ She must go.

Focus: must go

Ella debe ir.

Never add 's' or 'to' when using must.

#6 Mistake Corrected

✗ You mustn't pay → ✓ You don't have to pay.

Focus: don't have to

No tienes que pagar.

Use 'don't have to' for things that are free/optional, not 'mustn't'.

#7 Strong Recommendation

You must try the chocolate cake; it is the best in town!

Focus: must try

¡Tienes que probar el pastel de chocolate; es el mejor de la ciudad!

A very warm and strong suggestion to a friend.

#8 Advanced Usage

If you want to succeed, you must first believe you can.

Focus: must first believe

Si quieres tener éxito, primero debes creer que puedes.

Using must for a conditional necessity.

Test Yourself

Choose the correct form to complete the safety rule.

You ___ touch the oven. It is very hot!

✓ Correct! ✗ Not quite. Resposta certa: mustn't

We use 'mustn't' because it is dangerous and forbidden to touch the hot oven.

Complete the sentence about a personal obligation.

I ___ study tonight because I have a big test tomorrow.

✓ Correct! ✗ Not quite. Resposta certa: must

'Must' is followed directly by the base verb without 'to' or 's'.

Choose the best option for a logical conclusion.

Look at all that snow! It ___ very cold outside.

✓ Correct! ✗ Not quite. Resposta certa: must be

We use 'must be' when we are logically certain about a situation based on evidence (snow).

🎉 Score: /3

Visual Learning Aids

Mustn't vs. Don't Have To

Mustn't (Forbidden)
Danger Don't do it!
Illegal Against the law
Don't Have To (Optional)
Choice You can if you want
Free No payment needed

Should I use Must?

1

Is it a 100% required rule?

YES ↓
NO
Use 'should' for advice.
2

Is it in the past?

YES ↓
NO
Use 'had to'.
3

Is it negative?

YES ↓
NO
Use 'must'.

Where you see 'Must' in Real Life

✈️

Airport

  • • Must show ID
  • • Mustn't carry liquids
🏥

Hospital

  • • Must be quiet
  • • Mustn't use phones
💪

Gym

  • • Must wear shoes
  • • Must clean machines

Frequently Asked Questions

20 questions

It means something is 100% necessary or required. It is the strongest way to express an obligation in English, like in the sentence You must wear a seatbelt.

Yes, it is quite formal. You will see it often on signs, in legal documents, or in written instructions like All applicants must submit a photo.

No, must does not have a past tense form. If you want to talk about a rule from yesterday, you must use had to, such as I had to leave early yesterday.

They are very similar, but must usually comes from the speaker's feelings, while have to comes from outside rules. I must eat means I'm hungry; I have to eat might mean a doctor told me to.

No, it never changes. You say I must, you must, and she must. Adding an s like she musts is a common mistake.

No, never use to after must. The correct pattern is always must + base verb, like I must study.

Use mustn't when something is strictly forbidden or prohibited. For example, You mustn't park here means it is against the law to park.

No! Mustn't means 'don't do it,' while don't have to means 'you can do it if you want, but it's not required'. It's a huge difference!

Yes, but only for very strong advice. If you say You must try this cake, you are being very enthusiastic and friendly.

It is used for logical guesses. If you see someone with an umbrella, you can say It must be raining because you are almost certain it is true.

You put must before the subject, like Must I go?. However, this sounds very formal or even annoyed; most people ask Do I have to go? instead.

Yes, must not is just the full, more formal version of the contraction mustn't. Use must not in formal writing.

Yes, you can use it for future obligations. For example, I must finish my project by next Friday is perfectly correct.

It's a dialect difference. Americans prefer can't or not allowed to for prohibitions because mustn't sounds very old-fashioned or British to them.

Yes, much stronger. Should is a suggestion or a good idea, while must is a requirement or a rule.

Absolutely. Use it when you make a strong promise to yourself, like I must exercise more this year.

The most common mistake is saying must to. Just remember: must is a shortcut word that skips the to!

To be polite, use it for positive things like You must come over for dinner!. Avoid using it for commands to others unless you are their boss.

Yes, to show your determination. You can say I must say, I am very interested in this role to sound professional and certain.

Look at signs in public places. Whenever you see a 'No Smoking' or 'Keep Off the Grass' sign, try to say the rule out loud using mustn't.

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