The Complete Guide to Using Can, Could, May, Might, Must, Shall, Should, Will, Would

Modal verbs are auxiliary verbs that express necessity, ability, permission, or possibility. These special verbs (can, could, may, might, must, shall, should, will, would) dramatically change the meaning of main verbs and are essential for advanced English communication. Unlike regular verbs, modals don't add -s for third person singular, don't use 'do' in questions/negatives, and are always followed by base verbs.

1. Ability Modals: Can & Could

These modals express physical or mental capacity, with 'can' for present ability and 'could' for past ability or polite requests. They're also used to discuss theoretical possibilities and opportunities.

Present Ability (Can)

  • "I can speak three languages fluently." (current skill)
  • "She can run a marathon in under four hours." (physical capacity)
  • "Our team can solve complex problems efficiently." (collective ability)
  • "This software can process 1000 transactions per second." (functional capacity)

Past Ability (Could)

  • "When I was younger, I could touch my toes easily." (past capacity)
  • "He could read before he started kindergarten." (early developed skill)
  • "Before the accident, she could play piano beautifully." (lost ability)

Polite Requests & Possibilities

Requests:

  • "Can you help me with this heavy box?" (informal)
  • "Could you possibly lend me your notes?" (more polite)
  • "Could I bother you for a moment of your time?" (very polite)

Possibilities:

  • "We could go to the beach tomorrow if the weather's nice." (suggestion)
  • "You could try restarting your computer." (gentle advice)
  • "This could be the solution we've been looking for." (speculation)

2. Permission & Possibility Modals: May & Might

These modals express formal permission or uncertain possibility, with 'may' being more likely than 'might.' They're essential for polite communication and speculative discussions.

Asking/Giving Permission

  • "May I leave the meeting early today?" (formal request)
  • "You may begin the exam now." (official permission)
  • "Students may use calculators during the test." (authorized action)
  • "Might I suggest an alternative approach?" (extremely formal)

Expressing Possibility

May (50-70% likely):

  • "The CEO may announce the merger tomorrow." (good possibility)
  • "We may need to reschedule the event." (probable necessity)
  • "This discovery may change modern medicine." (significant potential)

Might (30-50% likely):

  • "It might rain later, so bring an umbrella." (uncertain prediction)
  • "She might know the answer to your question." (uncertain ability)
  • "We might consider other options if this fails." (contingent possibility)

Special Usage Notes:

  • In very formal contexts, "might" can express polite requests: "Might I ask your name?"
  • "May" can express wishes: "May you have a long and happy life!"
  • In reported speech, "might" becomes the past of "may": "She said she might be late."

3. Obligation & Necessity Modals: Must & Have To

These powerful modals express strong obligations, necessities, and logical conclusions. 'Must' indicates personal obligation or strong conviction, while 'have to' shows external requirements.

Strong Obligation (Must)

  • "All employees must wear safety goggles in the lab." (rule/requirement)
  • "You must see this new exhibition - it's incredible!" (strong recommendation)
  • "I must remember to call my mother today." (personal obligation)

Logical Deduction (Must)

  • "You've been traveling all day - you must be exhausted." (certain conclusion)
  • "This must be the correct address - I recognize the building." (high confidence)
  • "She's two hours late - something must have happened." (inevitable inference)

External Necessity (Have To)

Affirmative:

  • "I have to finish this report by 5 PM." (boss's requirement)
  • "You have to have a visa to enter the country." (legal obligation)

Negative (Don't Have To = No Obligation):

  • "You don't have to attend if you're not feeling well." (optional)
  • "We don't have to decide today." (no pressure)

Must vs. Have To Comparison:

"Must" "Have To"
Personal obligation: "I must exercise more." External obligation: "I have to pay taxes."
Formal rules: "Guests must sign in." General necessities: "You have to eat to live."
Logical certainty: "That must be painful!" Not used for deductions

Practice Exercises

Exercise 1: Choose the Correct Modal

  1. (Can/Might) you swim when you were five years old?
  2. All passengers (must/had better) fasten their seatbelts during takeoff.
  3. This (can't/mustn't) be John's house - he said he lives in a yellow one.

Exercise 2: Rewrite Using Modals

  1. Is it okay if I borrow your pen? → (May)...
  2. It's necessary for doctors to take the Hippocratic oath. → (Must)...
  3. I'm certain this is the right decision. → (Must)...

Exercise 3: Identify the Modal Function

For each sentence, identify if the modal expresses ability, permission, obligation, or possibility:

  1. "You must see a doctor about that cough."
  2. "Could I use your phone to make a quick call?"
  3. "She might know the answer to your question."

Advanced Usage Tips

  • For past ability, use "could" for general abilities but "was/were able to" for specific achievements: "I could swim as a child" vs. "I was able to finish the race"
  • "Shall" is becoming rare in American English but is still used in legal contexts and for suggestions: "Shall we begin?"
  • In very formal writing, "may" is preferred over "can" for permission: "Journalists may enter after the announcement"
  • For polite refusals, use "might" to soften: "I might not be able to attend" instead of "I can't attend"
  • In academic writing, "may" suggests possibility while "can" suggests capability: "This method may produce results" vs. "This device can measure temperatures"