You can use modal auxiliaries with the base forms of verbs to convey ideas of necessity, obligation, politeness, possibility and probability. Modals have only one form, so they are never spelled with an -s.
Must, could, might, may, couldn’t and can’t are used to show how possible or probable the speaker thinks a present situation is. Using “must” conveys a strong feeling of certainty; nevertheless, there is still some doubt in the speaker’s mind.
Could, may and might show that a speaker is 50% or less certain about a present situation.
By using couldn’t and can’t, a speaker shows that he believes a situation is very unlikely or impossible. Must not also conveys a strong feeling that something is not probable nor possible. However, unlike couldn’t and can’t, which are normally contracted, must not is usually written as two words.
Correct: You just ate three hamburgers, an order of large fries and dessert. You can’t still be hungry.
Incorrect: You just ate three hamburgers, an order of large fries and dessert. You mustn’t still be hungry.
May not and might not convey the idea that a speaker is 50% or less certain that a situation is not possible or probable. They are not normally used in their contracted forms.
Correct: They are late. They might not know the way here.
Incorrect: They are late. They mightn’t know the way here.
Might and could are used in questions regarding the possibility or probability of present situations. However, may and must are not.
Correct: Could Tom be in class now?
Incorrect: May Tom be in class now?
For most short answers, a modal alone is used. However, if the question includes a be verb, a modal with be is required.
Person A: Is Stephanie coming over tonight?
Person B: I’m not sure. She might be.
Person C: Does Rachel like spicy food?
Person D: I don’t know. She might.
Do not confuse may be and maybe. “May be” (two words) is a modal, whereas “maybe” (one word) is an adverb.
Maybe I will go to bed early tonight.
Example: I may be late getting home tonight.