Question

what is modals and what are its uses

Answer 1

Modals are  special kinds of verbs that are used before ordinary verbs to express meanings  such as permissions, possibility, certainty, necessity etc...
examples
1. can you lift this box?
2. shall I open the door?

Answer 2

Modals are a 'special' type of verb, which is considered to be auxiliary or helping.

USES:

It is used to express a the ability to do a work, the permission of doing a work or obligation or to discuss the possibility of doing a work. The modal phrases used to describe the same things as 'modals' but with a combination of both prepositions and auxiliary words.  

The modals in semi modals in English are:

• can
• could
• be able to.