Grammar Points: Modals & Modal Perfects
1. Ability & Permission
Can / Could: General ability. "I can speak English."
Be able to: Specific ability in a specific moment. "I was able to fix the car."
2. Obligation & Necessity
Must: Internal obligation or strong advice. "I must study harder."
Have to: External obligation (rules/laws). "We have to wear a uniform."
Needn't / Don't have to: Absence of obligation. "You needn't bring a gift."
3. Prohibition
Mustn't: It is forbidden. "You mustn't smoke here."
4. Advice
Should / Ought to / Had better: Giving advice. "You should see a doctor."
5. Possibility & Deduction
Must: 100% certain something is true. "He must be at home."
Can't: 100% certain something is impossible. "That can't be Peter."
May / Might / Could: 50% possibility. "It might rain later."
6. Modal Perfects (Past Speculation)
Must have + P.P.: Logical conclusion about the past. "She must have forgotten."
Can't have + P.P.: Impossibility about the past. "He can't have seen me."
Should have + P.P.: Regret or criticism. "I should have studied more."
May/Might have + P.P.: Weak possibility in the past. "He might have lost his keys."
Needn't have + P.P.: Action done but was unnecessary. "You needn't have cooked dinner."