Must

Must:

It expresses necessity:

Must is used to express that something is obligatory or very necessary.

Examples:

·        He must leave.

·        Ali’s driver must have license.

·         I must be at home at night. I am expecting an important telephone call at night.

‘Must’ can also be used to express strongly held beliefs, such as:

·         "It must be here somewhere" ,

 with the same meaning as: "I believe  it is very likely that it is here somewhere."

It expresses prohibition or resolution:

‘Must’ can be used to express a prohibition such as:

·         "You must not smoke in here".

or a resolution such as:

·         "I mustn't make that mistake again".

It expresses probability:

‘Must’ can be used to express probability.

Examples:

·         “All the lights in Fatima’s room are turned off. She must be sleeping.”

·        “Why is not Sara in the class? He must be sick.”

It must be noted that the speaker is not sure about the sickness of Sara, but he has some reason for making him believe that Sara is sick and that is, let us suppose, he saw her last night, and she was not feeling well.

Expressing probability in past:

·         “Sara was not in the class. She must have been sick.”

·         “I did not find her in her room last night. She must have been studying at the library.”

Use of ‘Must’ in negative forms:

In the negative forms: "must not" negates the main verb. In the sentence “You must not go", it is being expressed that it is obligatory for the person being spoken to not go.

Ought to:

It expresses advisability:

‘Ought to’ is used to express an ideal behavior or occurrence or suggested obligation, in a similar way to should:

Examples:

·         “You ought to lose some weight.”

·         “You ought to study harder.”

·         “You ought to respect your parents.”

 The negations are, respectively, ought not to (or rarely, oughtn't to).The negative forms negate the main verb:

·         "You ought not to do that"

·         "You ought to refrain from doing that".

Can be used for expressing expectation:

Examples:

·         “The bus ought to be here soon.”

·         “The bus ought to have been here 10 minutes ago (but it has not come whereas it was expected to be here 10 minutes ago).

In addition, ought to, like should, can be used to express relatively high probability, as in "It ought to rain today."