Modal Verbs 'must and ought to'

In this article we are going to study about the Modal Verbs ‘must and ought to’.

Uses of Must and Ought to

Use Case 1: Necessity or Obligation

Concept 1

In case of obligation (to say that it is necessary to do something) → We use ‘must’ or ‘have (got) to’.

Every guest in this party must / has (got) to be thoroughly searched.

To get a pass to the gala, you must / have (got) to know someone in the palace.

Concept 2

‘Must’ only has present tense form (no past tense, no participles, etc.).

So, ‘must’ refers to the present or the near future (not the past).
To talk about the past we use ‘had to’ (the past form of ‘have to’).

I must go to college. (talking about present)

Yesterday I had to go to college. (talking about past)

Concept 3

For future obligations (to say that it will be necessary for someone to do something in the future) → We can also use ‘will have to’, apart from ‘must’, ‘have (got) to’:

To complete my project, I must / have (got) to / will have to meet the professor.

Concept 4: ‘must’ Vs. ‘have (got) to’

When the obligation comes from the speaker → We generally use ‘must’.
When the obligation comes from somewhere else → We generally use ‘have (got) to’.

Compare:
I must start coding daily. (It is my own idea.)
I have to start coding on a daily basis. (my teacher has told me to code daily.)

Also, to talk about a necessity that is characteristic of a person → We prefer ‘have (got) to’:
Annu has got to have at least ten hours of sleep everyday.

Use Case 2: Conclusion

Concept 1

Conclusions can be positive or negative. It can be drawn in the present or in the past.

Conclusions being drawn in the present

When we conclude positively that something has happened or that something is true → We generally use ‘must’ (rather than ‘have (got) to’):

There must be around 10 thousand people at the carnival.

Conclusions being drawn in the past

To draw a positive conclusion about something in the past → We use ‘must + have + past participle’:

You must have been glad to watch him perform like that.

They must have played really well to win against such a strong team.

Concept 2

To give a negative conclusion → We use can’t (cannot) or couldn’t (rather than must not or hasn’t / haven’t got to)

Mr. Mose is the murderer? That can’t be right. He was with me then.

He wasn’t there at the time. He couldn’t have been the killer who stabbed Dwight.

Use Case 3: Urgency

Concept 1

Must is used to show urgency.

Your teeth are decaying, so you must consult a dentist.

Concept 2

We also use ‘must’ for strong recommendation.

If you want to score more than 90% marks in mathematics, you must read this book.

Use Case 4: Certainty

We also use ‘must’ to express logical certainty / strong possibility.

Living in such a tourist destination must be fun. (i.e. I am sure it is fun.)

Your father must be coming to town.

He must have left already. (I am sure he has left already).

Use Case 5: Probability

To express probability → We can use ‘ought (to) + $V_1$’

With increasing demand, the prices ought to go up. (probability)

This SEO course ought to be very useful. (probability)

Extra Books and Tools

If you prefer to learn via books, or want some good English Grammar books for reference purposes, you may read this article which enlists some of the books recommended by us.