Modal Verbs: must

Must with the indefinite 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)

Note: In negative form, must not is used to express a prohibition or obligation to not do something, just as in the present tense.

Example: You must not have told anyone about the surprise party. (obligation to not do something in the past)

In all of these cases, must is used to express a strong obligation, necessity, or deduction in the present or past tense.

Must with the Indefinite Infinitive Passive:

Must with the 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)

Have to vs. Must for past obligation:

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)

The 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.

Here are some examples of had to used in the past:

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.

For example: the sentence He must know her address

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.

This use of 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 the Continuous aspect:

When must is used with the continuous (progressive) aspect, it indicates an ongoing obligation or necessity that is happening now.

The construction is must be + present participle (-ing form).

For example: 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 behavior that is causing an ongoing problem:

The neighbors 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 the 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.

The construction is must have + past participle.

For example: 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 the 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.

Click here Modal Verbs: must

Click here Modal Verbs: to be

Click here Auxiliary Verbs: shall and will

Click here Auxiliary Verbs: to have

Click here Auxiliary Verbs: to be

Click here Auxiliary Verbs

Click here Auxiliary and Modal verbs

The Present Continuous tense

Modal Verbs

The Indicative Mood-the Passive Voice

The Future Continuous tense

The Basic forms of The English verb

Table of Irregular Verbs

Modal Verbs: may

The Past Perfect Continuous tense