Modal Verbs: Must and Should

1. Strong Obligation / Necessity (Rule or Law)

"Students must wear uniforms." (rule — no choice) "You must submit the form by Friday." (deadline — necessary) "All passengers must show their tickets." (requirement)

Difference from "have to": In many contexts, they are interchangeable. However, "must" is more forceful and often used for rules the speaker personally believes in.

2. Logical Deduction / Certainty

When you are almost certain something is true based on evidence:

"She hasn't eaten all day — she must be hungry." (logical conclusion) "He runs 10 km every morning — he must be very fit." "That must be your teacher arriving." (confident inference)

3. Prohibition (Must Not / Mustn't)

"You must not run in the corridors." (strong prohibition — do NOT do this) "Students must not use phones during exams."

Note: must not ≠ don't have to

"You must not go." = Forbidden (you cannot go) "You don't have to go." = Not required (but you can if you want)

1. Advice / Recommendation

"You should drink more water." (good advice, not a command) "She should see a doctor about that cough." "You shouldn't sleep late before exams."

2. Expectation / Obligation (Weaker than Must)

"Students should complete their homework." (expected, but not a strict rule) "She should arrive by 5 pm — she left an hour ago." (expected based on timing)

3. Logical Expectation

"The letter should be here by now." (I expect it to be here — but not certain)

Should have + past participle expresses regret about the past (you didn't do something you should have):

"You should have studied harder." (but you didn't — regret) "She shouldn't have said that." (she did say it — it was wrong)

Must have + past participle expresses certainty about a past event:

"He didn't answer — he must have been asleep." (logical conclusion) "She finished in 10 minutes — she must have practised a lot."

Choose must or should:

1. "You ___ brush your teeth twice a day." (advice) → should
2. "She hasn't slept — she ___ be exhausted." (deduction) → must
3. "Students ___ not carry mobiles into the exam hall." (rule) → must
4. "You ___ have called — I was waiting." (past regret) → should
5. "He ran out of breath — he ___ have sprinted the whole way." (past deduction) → must

1. What is the difference: "You mustn't eat this" vs "You don't have to eat this"?
2. Correct: "She should studied for the test."
3. Use "must" to express a logical deduction: "The floor is wet — ___."
4. Which is stronger advice: "should" or "must"?
5. Stretch: Rewrite using must or should: (a) "It's a rule: helmets are required." (b) "I recommend seeing the film." (c) "I'm certain — he went home early."