Modal verbs like MUST, CAN, and HAVE TO are used to indicate obligation, permission, and prohibition. They have specific grammatical rules: 1) Modal verbs only have one form for all subjects. For example, "you must study, she must study, he must study." 2) They don't use "do support" in questions or negatives, like "Must you study?" or "I mustn't eat ice cream." 3) They take the infinitive form of a verb without "to", such as "I must sleep well." The exception is "HAVE TO" which follows standard verb conjugation rules, adding "s" for third person and using "do

5.  MUST Permission
 CAN Obligation
 HAVE TO Prohibition

6.  MUST = Strong Obligation (MUSN’T =
Prohibition)
 CAN = Permission (CAN’T= Absence of
Permission)
 HAVE TO = Obligation (because I feel that I
need to do it) DON’T HAVE TO = absence of
obligation)

7.  1) Modal verbs are invariable: they have ONE
form for all the 1st, 2nd and 3d form.
 That means, you don’t add “S” in the 3d
person
 Examples:
 You must study. She must study. He must
study

8.  2) Modal verbs don’t have “DO support” in
interrogative and negative sentences
 Example:
 Must you study? Can I go now?
 I mustn’t eat ice cream because I’ve got flu.
 I can’t go out because my mom told me to
clean up my room

9.  3) After modal verbs you use INFINITIVE form
of a verb without TO
 Examples:
 I must sleep well
 I can swim

10.  4) Modal verbs don’t have Past Participle
form, Future Tense or ING form
 Can – only Can (in Past only could)
 Must – only MUST

11.  5) The exception is “HAVE TO”:
 A) It has “S” in the 3d form (She has to study
hard)
 B) HAVE TO has “Do support”: I don’t have to
study. Do you have to study?
 C) HAVE TO has Past Participle and Future
Tense: I had to study hard yesterday. I’ll have
to study hard tomorrow