Modal Auxiliary Verbs - Must


The modal verb must is most often used to express necessity—i.e., that something has to happen or be the case. We also use this sense of the word to indicate a strong intention to do something in the future, to emphasize something positive that we believe someone should do, and to rhetorically introduce or emphasize an opinion or sentiment. In addition to indicating necessity, must can be used to indicate that something is certain or very likely to happen or be true.


When must indicates that an action, circumstance, or situation is necessary, we usually use it in a declarative sentence. For example:
  • “This door must be left shut at all times!”
  • “We absolutely must get approval for that funding.”
  • “You must not tell anyone about what we saw.”
  • “Now, you mustn’t be alarmed, but we’ve had a bit of an accident in here.”
We can also use must in interrogative sentences to inquire whether something is necessary, usually as a criticism of some objectionable or undesirable action or behavior. For instance:
  • Must we go to dinner with them? They are dreadfully boring.”
  • Must you be so rude to my parents?”
  • Must I spend my entire weekend studying?”
However, this usage is generally reserved for more formal speech and writing, and isn’t very common in everyday English.

Indicating strong intention

We use the same meaning of must to indicate something we have a very strong intention of doing in the future. For example:
  • “I must file my taxes this weekend.”
  • “I must get around to calling my brother.”
  • “We must have the car checked out soon.”

Emphasizing a suggestion

We also use this meaning to make suggestions to others of something positive we believe they should do, as in:
  • “You simply must try the new Ethiopian restaurant on 4th Avenue—it’s fantastic!”
  • “It was so lovely to see you. We must get together again soon!”
  • “You must come stay with us at the lake sometime.”

As a rhetorical device

Finally, we can also use this meaning of must as a rhetorical device to politely introduce or emphasize an opinion or sentiment about something:
  • “I must say, this has been a most wonderful evening.”
  • “And I must add that Mr. Jones has been an absolute delight to work with.”
  • “I must be clear: we will disavow any knowledge of this incident.”
Note that we can accomplish the same thing by using the verbs let or allow instead, as in:
  • Let me be clear: this decision is in no way a reflection on the quality of your work.”
  • Allow us to say, we were greatly impressed by your performance.”

Certainty and likelihood

In addition to being used to indicate a necessary action or situation, must is also often used to describe that which is certain or extremely likely or probable to happen, occur, or be the case. For example:
  • “You must be absolutely exhausted after your flight.”
  • “Surely they must know that we can’t pay the money back yet.”
  • “There must be some way we can convince the board of directors.”
  • “I must have left my keys on my desk at work.”
  • Speaker A: “I just got back from a 12-week trip around Europe.”
  • Speaker B: “Wow, that must have been an amazing experience!”
Generally speaking, we do not use the negative of must (must not or mustn’t) to express a negative certainty or strong disbelief. Instead, we use cannot (often contracted as can’t), as in:
  • “You can’t be tired—you’ve been sleeping all day!”
  • “I can’t have left my phone at home, because I remember packing it in my bag.”
  • “After three years of college, she wants to drop out? She cannot be serious.”

1. Which of the following is not a function of the modal verb must?

2. In which of the following situations do we generally use can instead of must?

3. Which of the following sentences uses must to indicate a strong intention?

4. Identify the function of must in the following sentence:
“Jonathan, you must have driven through the night to have arrived so early!”

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