Verb must

Modal Verb /

Its Equivalent

Uses

Present / Future

Past

Must

(Must is used only in the present and future when the speaker decides. Must borrows the rest of its tenses from its equivalents

have to / be to.)

1. strong obligation /

(personal feelings

of necessity)

• We must follow the school; rules. (obligation; I am obliged to.)

• I must tell the truth. (I decide it is necessary.)

2. prohibition

◘ refusing

permission

(negative) /

◘ command

◘ emphatic request

• You mustn’t drive fast.

• You must not open that door.

3. 95% certainty

◘ possibility /

probability

◘ supposition

bordering on

assurance

• Mary isn’t in class. She must be sick. (present only)

• Mary must have been sick yesterday.

4. logical

assumption

• His face is red. He must be very angry. (I think he is very angry.)

• It can’t be true. It must be a lie.

(I think, I am fairly

sure; it’s logically

probable.)

• She couldn’t have lost her way; she must have missed the train. (= I don’t think she has lost her way; I think she has missed the train.)

Have to

(Have to is used when the necessity comes from outside the speaker or when others decide for him.)

1. strong obligation /

external necessity

• I have to go to class today.

• I have to do my homework every day. (Others decide it is necessary.)

• I had to go to class yesterday.

2. lack of necessity

(negative)

• I don’t have to go to class today.

• I didn’t have to go to class yesterday.

Have got to

(Have got to is more informal and usual than have to; is used for obligation on a single occasion.)

1. obligation /

necessity

• I have got to go to class today.

• I’ve got to leave early today.

• I had to go to class yesterday.

Be to

(Be to means must but it expresses the idea that someone else demands something.)

1. necessity

• I am to be here at 9.00. (My boss has told me to go there, there will be no one to meet Mr. Jones who is coming tonight.)

• I was to be here at 9.00.

Be to is used to express what someone expects about a previously planned / arranged event.

2. expectation

(about a planned /

arranged event)

• The conference is to start tomorrow. (It is scheduled.)

• The conference was to start yesterday. (It was scheduled.)

Be to is used to express something unavoidable (which is destined to happen).

3. something

unavoidable /

destined to

happen

• What is to happen to us?

• My dear father, if I am to get married, surely you will allow me to choose the time, place and person.

a. Present

b. Past

c. Future