#323: Should vs. Be Supposed To vs. Have To vs. Must | Talking about Obligation

Is there a difference among these sentences:  

• We must wait for our boss before we open. 
• We have to wait for our boss before we open.
• We are supposed to wait for our boss before we open.
• We should wait for our boss before we open. 

In each example, we’re using a modal of obligation—a verb that expresses necessity, advice, or a rule that’s not optional. But each modal communicates a different level of expectation, from personal advice to strict rules.

So, how do you know when to use Should, Be Supposed To, Have To, and Must?

In today’s lesson, we’ll break down the subtle but important differences, explore real-life scenarios, and learn how to avoid common mistakes. Plus, you’ll have the chance to practice with bonus questions to test your understanding.

Should vs. Be Supposed To vs. Have To vs. Must

‘Should’ for Mild Obligations

English speakers use the modal ‘should’ to express that there’s a mild obligation and/or to provide advice. 

Example

• You should start setting aside money for a rainy day fund.
• You shouldn’t smoke so much. It’s terrible for your health!*

*Note: We often use ‘shouldn’t’ to express that something is a bad idea or not socially accepted. 

For example, “You really shouldn’t eat so much chocolate; you’ll make yourself sick.”

‘Be Supposed to’ for Social Obligations

Similar to ‘should’ we use ‘be supposed to’ for obligations; however, the perspective is different. 

The modal ‘should’ expresses what you think is right, while “be supposed to” expresses what others think is right. It is used to describe stronger social norms, expectations, and unsaid rules.

• He was supposed to call me.
• You’re supposed to say “I”m sorry” when you’ve hurt someone’s feelings.
• We’re supposed to arrive by 10:00 AM. 

Let’s Compare

Take a look at the following sentences and determine the difference in meaning:

• Rosie should be here by 9:30 AM. (What I expect will happen.)
• Rosie is supposed to be here by 9:30 AM. (What is generally expected—not just by me but by others as well.)

On a side note, we also use ‘to be supposed’ to mean that something is likely to occur, someone is reputed to do something, or that the opposite was expected to happen. 

Examples

• The train is supposed to arrive at 11:30 AM.  (Likelihood)
• Mika is supposed to be the best pottery studio in town. (Reputation)
• It was supposed to rain today, but the sky looks clear. (Unmet expectation)

‘Have to’ for Strong Obligations

As a step up, we use ‘have to’ to express stronger obligations, usually due to an external force. Often, these obligations are general rules and impersonal. However, they can also be used for personal commitments. 

Example

• When they reach this age, children have to go to school. 
• You don’t have to work during the holiday season.* 
• I have to brush my teeth 2-3 times a day. 
• Do I have to wear a tuxedo for this event? 
• I have to remember to buy bread on the way home. 

*Note: We use the negative of ‘have to’ to mean that something is not required, necessary, or expected. However, it’s fine if you choose to do so. 

• For example, “I don’t have to wake up early on weekends.”

Before we continue to ‘must,’ let’s distinguish between the mild ‘be supposed to’ and the stronger ‘have to:’

• I’m supposed to be at the airport by 6:00 PM. 
• I have to be at the airport by 6:00 PM. 

ANSWER: (1) There’s an expectation to be at the airport by 6:00 PM. But this person may or may not take their time getting to the airport. (2) There’s no choice but to arrive before 6:00 PM. Otherwise, this person could miss their flight. 

‘Must’ for Stronger Obligations & Requirement

Finally, we use ‘must’ to express an even stronger obligation and necessity. Especially specific obligations concerning morals, the law, or a speaker’s firm opinion. 

We use ‘must’ to express an obligation due to a rule or law.

• Applicants must submit a copy of their immigration documents. 
• You mustn’t drive while under the influence of alcohol.*

*Note: We use ‘mustn’t’ to mean that something is prohibited, forbidden or banned by an authority

• For example, “Employees mustn’t share company accounts.”

We also use ‘must’ to express personal opinions and moral obligations based on principles or beliefs. 

• You must come to house for dinner on Friday. 
• You must address these concerns with your boss before it’s too late. 
• The kids must study hard to pass their exams. 

Past and Future Obligations

When expressing strong obligations we had in the past, we always use ‘had to’ or ‘didn’t have to.’ In fact, ‘must’ is not used with the past. 

Example

• We had to wear uniforms to school. 
• I didn’t have to submit additional documents. 

We do not say, “When I was a child, we must wear uniforms to school.” This would mean the obligation exists in the present. 

When expressing future obligations, we can use ‘must’ or ‘will have to.’

• We will have to/must apply for an e-visa once we buy our plane tickets. 

Lastly, English speakers use ‘should have to’ to express regret about mild obligations that weren’t acted upon in the past or share advice on a better past action. 

Example

• You should’ve gone to bed earlier if you wanted to catch the earliest bus. 
• I shouldn’t have taken that job; I was too young and naive. 

Bonus Practice 

Fill in the blanks using a modal from today’s lesson and continue the sentence. You can share your answers in the comments below.

1. I _____ take a vacation to reduce stress and…
2. According to my boss, I _____ relocate for this promotion but…
3. My parents _____ exercise more to improve their…
4. I _____ call my friend to…

All my best,

~ Annemarie

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