Must vs. Have To: How to Know When to Use Them

It can be tricky to know when to use must and when to use have to , as both terms have a similar meaning: to express a need or obligation.

However, must and have to also have their own meanings. Must is a modal verb, whereas have to is an auxiliary verb. But you can figure out when to use must or have to by asking and answering seven simple questions.

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1. Is the Obligation Internal or External?

Must and have to both express obligation (a need). However, it makes a difference whether that need is external or internal. Let me explain with some examples.

If the obligation is external (meaning someone else gave the command) you should use have to.

For example:

I have to finish my homework because my teacher told me to.

In this case, the speaker didn’t decide to do their homework because they wanted to, but because the teacher instructed it. This is an external obligation.

Here’s another example:

Dad said you have to eat all your vegetables.

In this example, the decision to eat vegetables is motivated by the external obligation given by the dad. In both of these examples, the need is motivated by outside pressure and consequences.

On the other hand, if the obligation is internal (meaning the speaker made the decision) you can use must. Let’s look at a few examples:

I must study so I pass the test.

I must eat this because I want to be healthier.

As you can see, the speaker is self-motivated in these examples.

This isn’t a hard and fast rule, but it can help you decide whether you should use must or have to.

You probably won’t hear anyone use must when talking about outside obligations, but you will hear people use “have to” for both internal and external obligation.

You’re just as likely to hear either of these sentences:

I must leave before it gets dark.

I have to leave before it gets dark.

So, to summarize, if the need is motivated by someone else, you should use “have to.”

If the need is self-motivated, you can use either “have to” or “must.”

2. Are You Using Formal or Informal English?

You probably know that English, like many languages, can be formal or informal. Depending on the situation, you might use different words and phrases in order to sound more polite.

For example, when you’re at school, at work, talking to someone older than you or meeting someone new, you should use formal English.

However, when you’re with your friends or family it’s okay to switch to informal speech.

You wouldn’t greet your friend by saying: “Hello, how are you doing today?” Instead, you might say: “Hey, what’s up!”

So, where do must and have to fit in with formal and informal English?

In general, “have to” is less formal than “must.” You might use have to as a command. For example:

Henry has to do the dishes every Wednesday.

You have to go to detention after school.

Have to can also be used as a request in informal settings.

Are you busy tonight? You have to help me get ready for my date!

If this is a conversation between friends, it’s more of a request than a command. There aren’t consequences if the friend doesn’t help. However, this can be a way to add urgency or emphasis.

Must is more often used in formal English. It isn’t as commanding as have to, but instead implies polite insistence. For example:

You must forgive me.

You must come for dinner tonight.

To help you remember that must is more formal, you can remember this love quote from Jane Austen’s “Pride and Prejudice”: “You must allow me to tell you how ardently I admire and love you.”

3. Do You Need to Add Emphasis?

You can decide whether or not to use must or have to based on if you want to add emphasis. Let’s dig in a little deeper.

We already mentioned that must can be used in a formal setting to show polite insistence. In a similar way, you can also use “must” to show a sense of urgency, while still being polite.

I must get going.

In this case, you’re adding emphasis on your need to leave, but also that you don’t really want to. It’s as if you’re saying that you wish to stay but have determined you shouldn’t.

However, if you were to say:

I have to get going.

It doesn’t sound as insistent as using “must.” It’s less polite and doesn’t have as strong of an emphasis.

If you say something like “I have to leave,”  it comes across a little different. You might be emphasizing the fact that there will be consequences if you don’t leave. For example, it could be extended into:

I have to leave, or my mom will be angry.

4. Are You Making an Inference?

If you’re trying to make an inference, you need to use must. You can express a guess by using must if you have evidence to back up your guess.

Here’s what this looks like in a sentence:

You must be so tired after working all night.

She must be sad that her bike was stolen.

You can’t use “have to” in this way.

Incorrect: You have to be so happy that your niece was born.

Correct: You must be so happy that your niece was born.

5. Are You Asking a Question?

When you’re asking a question, it’s much more common to use have to than it is to use must. When you make a question with have to, add the verb “do.”

For example:

Do you have to sing every time we watch this movie?

Why does Anna have to come over?

When do I have to turn in my essay?

If you use must in a question, it’ll sound much more formal and often expresses frustration or exasperation.

Here are some example questions using must:

Must he wear that yellow vest again?

Must you sing so loudly?

As you can see, the questions using must sound more aggressive or annoyed. They also sound more formal than questions using have to.

Finally, if your question uses the verb “to do,” you need to use have to instead of must.

Incorrect: Do you must eat pickles?

Correct: Do you have to eat pickles?

Correct: Must you eat pickles?

6. Are You Using the Negative Form of Must or Have To?

An interesting thing happens when you use the negative versions of must and have to. When you use the negative form of “must”— must not —you’re expressing absolute obligation.

You must not eat these cupcakes.

He must not be late to work.

In the negative form, must not doesn’t leave any room for questions. It’s a command, not a suggestion.

However, the negative form of “have to” ( not have to ) is optional.

He does not have to turn in his homework assignment.

You do not have to go to the park.

These statements aren’t absolute. Instead, they’re more of a suggestion or an allowance.

You do not have to go to the park, but you can.

He should turn in his homework assignment, but he does not have to.

When using the negative form of have to, we usually use contractions.

She doesn’t have to cut her hair.

You don’t have to wear makeup.

You can also use a contraction for the negative of “must not” ( mustn’t ), though it’s less common and may sound archaic (old-fashioned).

You mustn’t tell a soul.

She mustn’t wear that dress.

7. Which Tense Are You Using?

Must is most commonly used in the present tense.

You must stop right now.

It can also be used to talk about something that needs to happen in the future.

You must teach me archery next month.

You can also use must to talk about something in the past, but you need to use “must have.”

Here are examples:

Sam must have left it at school last week.

Abraham Lincoln must have been very brave.

Have to can be used in the past, present or future tenses. You only need to conjugate the verb have as you would normally.

I had to take a nap yesterday.

I have to hang up the phone.

I will have to go to the doctor tomorrow.

 

These questions can help you determine when to use must vs. have to. When studying these differences, try writing several example sentences for each question above.

By thinking through each of these questions, you should be able to use the correct word in any situation. And by mastering tricky phrases like these, you’ll sound more like a native speaker in no time!

While these questions are outstanding starting points, mastering must vs. have to all boils down to practice, practice and more practice. Participate in or listen to a lot of real English conversations. That way, you can see which word would be more commonly used for what situation. 

If no English speaker is around, then you can listen to English content like podcasts, interviews and movies. You can also use a language learning program like FluentU.

With enough review, you’ll know which one you should say without even thinking of all these rules!

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