Grammar Rule: No Changes!
Modals never change. No "s", "ing", or "ed".
Subject + Modal + Verb (base)
"He must go." (NOT: He musts go)
Can (Ability & Permission)
Ability: "I can swim."
Permission: "Can I help you?"
Negative: "I can't drive."
Must (Obligation)
Obligation: "You must wear a seatbelt."
Prohibition: "You mustn't shout."
Note: Mustn't means it is forbidden.