Modal verbs (or modals) are special auxiliary verbs that change the meaning of the main verb. They express concepts like ability, possibility, permission, obligation, and deduction. They are essential for conveying attitude, politeness, and degrees of certainty.

Core Modal Verbs:
can, could, may, might, will, would, shall, should, must

Semi-Modal Verbs (Phrasal Modals):
ought to, have to, need to, be able to, be allowed to, be supposed to, be going to, used to, had better, would rather

Rule 1: Followed by Base Verb: Modals are always followed by the base form of a verb (infinitive without 'to').
Correct: He can speak English.
Incorrect: He can to speak English / He can speaks English.

Rule 2: No '-s' for Third Person: Modals do not change form for he, she, or it.
Correct: She must leave now.
Incorrect: She musts leave now.

Rule 3: No Double Modals: You cannot use two core modal verbs together.
Correct: He will be able to come.
Incorrect: He will can come.

Rule 4: Forming Questions and Negatives: To make questions, invert the modal and the subject. To make negatives, add 'not' after the modal.
Statement: You should go.
Question: Should you go?
Negative: You should not (shouldn't) go.

Modals serve many different functions depending on the context.

A. Ability (can, could, be able to)
Present/Future: She can speak three languages.
Past: He could run very fast when he was young.
For other tenses: I have been able to solve the issue.

B. Permission (can, could, may)
Informal: Can I borrow your pen?
Polite: Could I ask a question?
Formal: May I come in?

C. Advice (should, ought to, had better)
General Advice: You should see a doctor.
Moral Duty: You ought to respect your elders.
Strong Advice/Warning: You had better finish your homework, or you'll be in trouble.

D. Obligation & Necessity (must, have to, need to)
Internal Obligation (speaker's feeling): I must finish this today.
External Obligation (a rule or law): You have to wear a uniform at school.
Necessity: I need to buy some groceries.

E. Possibility (may, might, could, can)
General Possibility: It might rain tomorrow. (less certain)
Slightly more certain: It may rain tomorrow.
With specific conditions: We could go to the park if the weather is good.
General truth: It can be very cold in winter.

F. Deduction (must, might, may, could, can't)
Certainty (Positive): The lights are on. She must be home.
Possibility: He isn't answering. He might/may/could be in a meeting.
Certainty (Negative): He just left. He can't be home already.

Perfect modals are used to talk about past actions or states, often with a sense of speculation, regret, or criticism.

A. Past Deduction
Certainty (Positive): The ground is wet. It must have rained last night.
Possibility: She's late. She might have missed the bus.
Certainty (Negative): He looks happy. He can't have failed the exam.

B. Regret or Criticism
Regret/Criticism: You should have told me the truth. (But you didn't)
Stronger Regret: You ought to have been more careful.

C. Lost Opportunities / Past Possibilities
Unrealized Past Ability: I could have gone to the party, but I was tired.
Unrealized Past Willingness: I would have helped you, but I didn't know you needed it.

A. Conditional Sentences
Type 2 (Hypothetical Present): If I were you, I would study harder.
Type 3 (Hypothetical Past): If I had known, I would have come.

B. Politeness and Formality
Modals can make requests, offers, and statements more polite. Generally, past forms are more polite.
Less Polite: Can you help me?
More Polite: Could you help me?
Very Polite/Formal: Would you mind helping me?

C. Passive Voice
The structure is: modal + be + past participle.
Example: The report must be finished by 5 PM.
Example: This door should be kept closed.

D. Reported Speech
Modals often change to their past forms in reported speech.
Direct: She said, "I can swim." -> Reported: She said that she could swim.
Direct: He said, "I will call you." -> Reported: He said that he would call me.

Semi-modals are phrases that act like modals but use auxiliary verbs like 'be' or 'have'. They are useful because they can be used in more tenses.

Have to: Strong external obligation.
Examples: I have to work on Saturday. Do you have to leave?

Be able to: Ability, can be used in all tenses.
Examples: I won't be able to attend the meeting. She has been able to swim since she was five.

Be supposed to: Expectation or duty.
Examples: You are supposed to start work at 9 AM. What am I supposed to do?

Had better: Strong advice or warning for a specific situation.
Examples: You had better hurry or you will miss the train. You'd better not forget.

Would rather: Preference.
Examples: I'd rather have tea than coffee. Would you rather stay in or go out?

Using 'to' after a modal: Incorrect: `I must to go.` -> Correct: `I must go.` (The only exception is 'ought to').

Adding '-s' to the modal: Incorrect: `She cans sing.` -> Correct: `She can sing.`

Using a modal with 'do/does/did': Incorrect: `Do you can come?` -> Correct: `Can you come?`

Confusing 'must not' and 'don't have to': `Must not` means prohibition (it's forbidden). `Don't have to` means no obligation (it's not necessary).

Fill in the blank with the correct modal: You look tired. You ______ get some rest. (should/must)

Rewrite the sentence using a perfect modal: It's possible that he forgot the appointment. -> He ______ forgotten the appointment.

Choose the correct option: You (must not / don't have to) pay to enter the museum; it's free.

Make a polite request: You want to borrow your friend's car. -> ______ I possibly borrow your car this weekend?