Modal auxiliary

Modal verbs are used for ideas and possibilities. Modal verbs in English are auxiliaries: we put them with other verbs.
She can speak three languages.
It could rain tomorrow.
I must go.
In these sentences, we put can with speak, could with rain, and must with go.

The three most common functions for modal verbs are ability, probability, and obligation.

She can speak shows ability: she has the ability to speak.

It could rain shows probability: the chance of rain is small.

I must go shows obligation: I have a need to go.