Question

what are modal in english grammar

Answer 1

A modal is a word which expresses the mood of the verb. It originated from Latin (modus). Brainstorm 12

Answer 2

Modals or modal verbs are such verbs which are defining the mood in the sentence. However, it's the mood which should be understood in a bit wider scope, such as possibility, probability, obligation, proposition, request, etc. 

In English, modals are mainly use to alter the basic meaning of a sentence and they cannot function on their own, that is why they are also called auxillary verbs.

Here you've got a list of the most popular modals, if not all of them: 

can | could
shall | should
may | might
will | would
ought to 
must

I hope it was helpful ;)