The word “must” means more than just telling people what is necessary.
Most people know the word “must” is a way of telling someone that something is necessary, such as when you tell someone “you must wear a seatbelt when you drive.” This is a correct and common use of “must”, but there is more than one way to use this important modal.
Another way to use “must” is to express to someone that you are very confident about how they feel, even though you are not experiencing their situation or those emotions yourself.
For example, if your friend tells you that they did not sleep last night, you could express that you are confident that your friend feels tired today, even though you did not have this experience and you do not have this tired feeling yourself. This is empathy, which is showing someone that you can understand their feelings based on a certain situation, even though you are not in that situation.
If you are confident that your friend feels tired based on what they just told you, you can use the modal “must” with the “be” verb and the adjective “tired” to express empathy and understanding of their situation:
"You must be tired.”
The “be” verb is always the word “be” after a modal like “must” (not “is”, “are”, or “am”), and we need the “be” verb in this case because we are adding an adjective. If it was a regular sentence, it would be “you are tired” but we are using the modal “must” so it becomes “you must be tired”.
When you tell someone “you must be tired” it means that you can confidently predict their feeling based on what you’ve heard from them.
The reason that we have to use “must” is because if you do not use “must” and say: “you are tired” it means you are saying a fact about someone else’s feelings even though you don’t know if this is their feeling yet. For example, if you tell me that you got a new job, I cannot say to you: “you are happy”, because this is telling you how you feel as a fact, when I have not confirmed this fact yet. However, I can say: “you must be happy!” which means that I am confidently predicting your feeling (happy) based on what you just told me.
After someone tells you specifically that they are happy or tired, then you can say: “you are tired/happy”, because now they have confirmed it. On the other hand, if we do not have this confirmation yet, then we use this expression to show that we understand how they feel even without them telling us.
Look at the following dialogues, where “Person B” responds with “you must be (adjective)” to show empathy based on what they heard from “Person A”.
Person A: “I’m finally going on my trip to Asia tomorrow!”
Person B: “You must be excited!”
Person A: “I lost my dog temporarily, but we found her in our neighbor’s yard.”
Person B: “You must be relieved!”
Person A: “I saw the perfect car a few days ago, but I found out today that it was already sold .”
Person B: “You must be disappointed.”
In each example above, Person B is predicting how Person A feels after hearing the story from Person A. Note that the words are adjectives to describe feelings: “excited”, “relieved”, and “disappointed.”
If you want to use an adjective to describe the situation instead of the feelings of the person, you need to use the subject “it” to represent “the situation”. If you take the previous examples and change the subject from “you” to “it” (the situation), then the adjective can describe the situation (ex. “exciting” instead of “excited”):
Person A: “I’m finally going on my trip to Asia tomorrow!”
Person B: “It must be exciting!”
Person A: “I lost my dog temporarily, but we found her in our neighbor’s yard.”
Person B: “It must be relieving!”
Person A: “I saw the perfect car a few days ago, but I found out today that it was already sold.”
Person B: “It must be disappointing.”
Finally, if you are talking about someone’s feelings in the past, or a situation that is already finished, you need to use a past form of language. When you are using a modal like “must” to describe the past, you need to use present perfect and change “be” to “has/have been”.
For example, if you think someone is feeling tired today, you can say: “you must be tired” but if someone described their situation a few days ago and you want to say that you are confident that they felt tired at that time (a few days ago), you have to say: “you must have been tired!”
Look at the examples below and how Person B uses “must + present perfect” to describe the past:
Person A: “I had to answer 100 questions in one hour during the test!”
Person B: “It (the test) must have been difficult!”
Person A: “My husband tried ice skating for the first time. He kept falling everywhere.”
Person B: “It (the situation) must have been funny.”
Person A: “I accidentally walked into the wrong washroom.”
Person B: “You must have been embarrassed!”
To learn more about Modals like "must", "could", "should", and "would", read the book "Grammar Essentials"
Practice
Imagine that someone said to you the sentences 1 – 6 below. How would you respond using “must” to show empathy to that person? Use the adjectives below to help you, but be careful about whether the subject should be “you” or “it” and whether the time should be present “must be” or past “must have been”:
Boring / Nervous / Scared / Hungry / Delicious / Stressful
I didn’t eat any breakfast this morning.
I have a big university exam tonight.
My sister prepared my favorite seafood for my birthday dinner last weekend!
I’m moving to a new house next week and I have to prepare all my things alone.
Last night, I thought that a car was following me while I was walking home.
We had no electricity yesterday so I couldn’t do anything all day in the house.
Answers
You must be hungry.
You must be nervous.
It must have been delicious! (“it” means the food)
It must be stressful (“it” means the situation, and the adjective “stressful” describes a situation)
You must have been scared.
It must have been boring.
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