Must, should, ought compared

• All the three verbs serve to express obligation.

• Must, however, sounds more forceful, peremptory.

e.g. You must do it at once. (Вы должны (обязаны) сделать это немедленно.)

• Both should and ought to express obligation, advisability, de­sirability and are used when must would sound too peremptory.

e.g. You should do it at once. You ought to do it at once.

(Вам следует( нужно) сделать это немедленно.)

• Should and ought to are very much alike in meaning and are often interchangeable.

• ought to- moral obligation

• Should is com­mon in instructions and corrections.

e.g. You ought to help him; he is in trouble.

You should use the definite article in this sentence.

• Notice that ought to cannot be used instead of emotional should.

• Must, ought to and should serve to express supposition implying strong probability.

• Must, however, seems to be in more frequent use than the other two verbs.

should + Perfect Infinitive, ought to + Perfect Infinitive and was/were to + Perfect Infinitive Compared

• Should + Perfect infinitive and ought to + Perfect infin­itive show that the action has not been carried out though it was desirable;

• was/were to + Perfect infinitive indicates an action that has not been carried out though it was planned. '

e.g. You should have helped him. (Now he is in trouble.)

You ought to have warned him.

He was to have arrived last week. (But his plans were upset by some cause.)