Out of all the verbs, modal verbs form a small, special group. First of all, modal verbs are not 'doing' words. They don't describe a definite action that someone is doing. Most ordinary verbs are used by themselves. They don't need anything else to help them express their meaning. But modal verbs are used with another verb, not on their own.
What does a modal verb do if it does not describe an action? A modal verb is used to express ideas such as necessity, ability, intention and possibility. Look at the five sentences below. There is no actual action involved in the last four. Nobody is actually doing anything.
Can you see how these sentences differ from the first one? The last four sentences all use a modal verb. They are expressing an idea not an actual action.
I am going to bed early tonight.
You 'must' go to bed early tonight. (Necessity)
You 'can' go to bed early tonight. (Ability)
'Will' you go to bed early tonight? (Intention)