Must is the most universal modal verb expressing obligation, - it can practically express all kinds of obligation, from high moral obligation to everyday necessity.

The captain said that we must save the man's life.
Капитан сказал, что мы должны спасти жизнь этого человека.

I must call in at a bakery.
Я должен зайти в булочную.

Must has but one form which can denote an action referring to the present, sometimes to the future.

I must call him tomorrow.
Я должен позвонить ему завтра.

When the action refers to the past, must is replaced by the modal verbs expressing obligation depending upon the nature of obligation.

  • The result of previous agreement to be.

    I was to meet Kate at the station.
    Я должен был встретить Кейт на остановке.

  • Necessity - to have.

    I had to stay after work to help him.
    Мне пришлось остаться после работы, чтобы помочь ему.

  • Moral obligation - should.

    You should have told him about it yesterday.
    Ты должен был сказать мне об этом вчера.

Note: One should be careful in using must in negative and interrogative sentences.

  • In negative sentences must express very strict prohibition and shouldn't be used unless the speaker is afraid to sound very categorical.

    Visitors must not feed the animals. (announcement in a Zoo)
    Посетителям запрещается кормить животных.

  • In interrogative sentences must shouldn't be used unless the speaker wants to show that he does not feel like performing the action.

    Must I do it right now?
    Что, прямо сейчас мне это сделать?

Besides the obligation must can also express somebody's supposition bordering on assurance.

You must be right.
Должно быть ты прав.

He must be still sleeping.
Должно быть он еще спит.

He must have left.
Похоже он ушел.

They must have been waiting for us since the very morning.
Должно быть они нас ждут с самого утра.

In secondary modality must can be used with all tense forms of the infinitive:

The Active voice The Passive voice
Indefinite to write to be written
Continuous to be writing
Perfect to have written to have been written
Perfect Continuous to have been writing

The Indefinite Infinitive denotes an action referring to the present or future, the Continuous Infinitive - an action going on the present moment, the Perfect Infinitive - an action referring to the past and the Perfect Continuous Infinitive - an action which began in the past, has been going on for a certain period of time and still going on now.