Modal Verbs: must
Must
Must with Indefinite Infinitive:
Must with indefinite infinitive is used in the present or in the future to express a strong obligation, necessity, or deduction.
Examples: I must study for my exam. (Strong obligation)
You must be tired after working all day. (Deduction based on evidence)
We must be at the airport by 6 am. (Necessity)
Note: In negative form, must not is used to express a prohibition or obligation to not do something.
Example: You must not forget to bring your passport to the airport. (Prohibition)
Must with Indefinite Infinitive Passive:
Must with indefinite infinitive passive is used to express a strong obligation or necessity for something to be done in the passive voice.
Examples: The report must be completed by Friday. (Strong obligation for the report to be completed)
The guests’ needs must be attended to at all times. (Necessity for the guests’ needs to be attended to)
Note: In negative form, must not is used to express a prohibition or obligation to not do something in the passive voice.
Example: The patient’s privacy must not be violated by anyone. (Obligation to not violate the patient’s privacy)
In all of these cases, must with the indefinite infinitive passive is used to express a strong obligation, necessity, or prohibition for something to be done in the passive voice.
Must and Have to:
Have to is often used as a substitute for must to express obligation, but there are some differences in usage.
Have to vs. Must for obligation:
Have to is used to express a general obligation, while must is used to express a more urgent or authoritative obligation.
Examples: I have to finish my homework tonight. (General obligation)
I must finish this report by tomorrow morning. (Urgent obligation)
Had to vs. Must for past obligations:
Had to (have to) is used to express past obligations that were external or imposed by circumstances, while must is used to express past obligations that were internal or self-imposed.
Examples: I had to study all night for the exam because I hadn’t prepared well. (External obligation)
I must have done something wrong to have failed the exam. (Self-imposed obligation)
Have to vs. Must for negation:
Must not is used to express prohibition, while do not have to is used to express lack of obligation.
Examples: You must not smoke in the building. (Prohibition)
You do not have to come to the meeting if you’re busy. (Lack of obligation)
Past form of Must is Had to:
Had to is used to express past obligation or necessity, while must is used to express present or future obligation.
Examples: I had to work late yesterday. (Past obligation)
He had to study hard to pass the exam. (Past necessity)
We had to cancel our trip due to bad weather. (Past circumstance)
Note that had to is also used in the negative form, did not have to to express the lack of obligation or necessity in the past: I did not have to attend the meeting yesterday. (lack of obligation)
She did not have to take the bus because her friend gave her a ride. (lack of necessity)
When must expresses probability in the present, it is used to indicate a strong inference or assumption about a present situation.
Example: the sentence He must know her address, which suggests that the speaker believes it is very likely that he knows her address, based on the information available or some logical deduction. The speaker is not certain but is making a strong assumption based on the context.
Must is often used in combination with adverbs such as probably or likely to further indicate the degree of probability.
Example: He’s lived in this town his whole life, so he probably knows her address. He must know her address by now.
Must with Continuous Infinitive:
When must is used with the continuous (progressive) aspect, it indicates an ongoing obligation or necessity that is happening now.
Must be + present participle (-ing form).
Examples: I must be working on this project right now. (I have an ongoing obligation to work on the project at this moment.)
She must be studying for her exam. (It’s necessary for her to be studying for her exam currently.)
They must be waiting for us at the restaurant. (It’s likely that they’re waiting for us because we made plans to meet there.)
This use of must can also be used to express irritation or frustration about someone’s behaviour that is causing an ongoing problem:
The neighbours upstairs must be renovating their apartment again. (I’m annoyed by the noise and disruption caused by their ongoing renovations.)
My boss must be expecting me to work overtime again. (I’m frustrated by the ongoing expectation to work extra hours.)
Must with Perfect Infinitive:
When must is used with the perfect infinitive, it indicates a past obligation or necessity that was completed or resolved in the past.
must have + past participle.
Examples: She must have finished her presentation by now. (I assume that she has completed the presentation because it was due earlier.)
They must have arrived at the airport already. (It’s likely that they have arrived because their flight was scheduled to land earlier.)
He must have lost his phone. (I infer that he lost his phone because he’s been trying to call me from another number.)
This use of must with perfect infinitive suggests a strong inference or assumption about a past event.
It’s often used in combination with adverbs such as probably or likely to further indicate the degree of probability.
Example: They were supposed to finish the project yesterday, so they must have completed it by now.
Must with perfect infinitive can also be used in the past to express a strong obligation, necessity, or deduction that was true in the past.
Examples: He must have forgotten his wallet at home. (Deduction about the past)
They must have worked very hard to finish the project on time. (Necessity in the past)
I must have left my keys in the car yesterday. (Deduction about the past)
Auxiliary Verbs: shall and will