Modals are a type of auxiliary verb. They are used with the main verb to express possibility or necessity.
There are a number of modals, which express different levels of possibility or necessity:
We can use shall to make suggestions, talk about the future, express certainty, and issue commands.
We can use should to make recommendations and give advice, express obligation, and express expectation.
Using Ought To
We use ought to in the same way as we use should: to make recommendations and give advice, express obligation, and express expectation.
We use could to make requests and suggestions, and to express possibility. It is also the past form of can.
We can use might to express possibility and uncertainty, make polite requests, and as the conditional form of may.
We can use must to express obligation or necessity, express certainty, and express prohibition.
We can use would as the past form of will, to talk about future events that are now in the past, and in polite phrases.