Let us now learn about some other Modal verbs and learn the use of must and ought to.

MUST, OUGHT TO

Must is used to express necessity or obligation. For example in sentences :

1. You must Improve your spelling.
2. We must get up early.
3. Must is expressing a necessity.

Also, Must refers to the present or the near future. It is important to note that must has no past form. Must is often used when the obligation comes from the speaker. However when the obligation comes from somewhere else, �have to� is often used. For example

I must be on a diet. (Here we are using �must� as this obligation comes from the speaker of the sentence)
I have to be on a diet. (Here we are using �have to� as this obligation comes from somewhere else, apart from the speaker of the sentence)

Must can also express logical certainty.

For example: Living in such crowded conditions must be difficult. Here �Must� is expressing a logical certainty of the difficulty of living in a crowded condition.

Ought (to) expresses moral obligation or desirability; for example
1. We ought to love our neighbors.
2. We ought to help him.
3. You ought to know better.

Ought (to) can also be used to express probability. For example
1. Prices ought to come down soon.
2. This book ought to be very useful.

Read More: What is a Modal Verb? Use of Shall, Should, Will, and Would