Might, May, Could or Must? Talk About Maybe Like a Native
Quick answer: Use might, may or could when you are not sure, must when you are almost sure something is true, and can't when you are almost sure it is not. Follow them with the base verb, and use have + past participle for guesses about the past.↗ Share on X
To talk about possibility in everyday English, use might, may, or could when you are not sure ("It might rain"), must when you are almost sure something is true ("You must be tired"), and can't when you are almost sure something is *not* true ("That can't be right"). After all of them, use the base verb — no "to," no "-s." For the past, add have + past participle: "She might have missed the bus." That is the whole system. The rest of this article shows you how native speakers really use these words, with examples you can copy today.
What are modals of possibility?
Transition Words for Work Emails That Don't Sound Stiff →
English Conditionals for Talking About Your Career Plans →
Gerund or Infinitive? 4 Rules for Everyday English Speech →A modal is a small helper word that goes before a main verb. Modals of possibility show how sure you are about something. They do not show facts. They show your guess.
Think of it as a scale from 0% to 100% sure:
| How sure you are | Modal | Example |
|---|---|---|
| Almost 100% sure it's true | must | He must be at work. His car is gone. |
| Fairly sure | should | The package should arrive today. |
| Maybe (50/50 or less) | may | I may be late tonight. |
| Maybe (a little less sure) | might | It might snow this weekend. |
| It's possible | could | That could be your phone ringing. |
| Almost 100% sure it's NOT true | can't / couldn't | She can't be 40. She looks 25! |
| Fairly sure it's NOT true | might not / may not | He might not come to the party. |
The percentages are not exact. Native speakers do not calculate. But this order helps you choose the right word quickly.
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How do you build a sentence with a modal?
The pattern is always the same:
Subject + modal + base verb
1. I might go to the gym later.
2. She could be right.
3. They must know the answer.
Three rules beginners often break:
- No "to" after the modal. ✗ "I might to go." ✓ "I might go."
- No "-s" on the verb, even with he/she/it. ✗ "He might goes." ✓ "He might go."
- No "-s" on the modal. ✗ "She musts be tired." ✓ "She must be tired."
For actions happening right now, use modal + be + -ing:
- "Don't call him. He might be sleeping."
- "Listen! Someone must be playing the piano."
What is the difference between might, may, and could?
Phrasal Verbs for Work Emails: 30 That Sound Natural →
'I Forgot' in English: 8 Common Mistakes and Natural Fixes →
Countable vs Uncountable Nouns: Fix Your Everyday Mistakes →In daily conversation, these three are very close. You can often switch them without changing the meaning much.
- Might is the most common in casual speech. Use it with friends, family, coworkers. "I might order pizza tonight."
- May sounds a little more formal or polite. You will see it in emails, news, and announcements. "Flights may be delayed due to weather."
- Could often suggests one option among several. "Where's my wallet? It could be in the car, or it could be in my jacket."
Important: "Could" for possibility is not the same as "could" for past ability.
- Possibility: "That could be a problem." (maybe now or in the future)
- Past ability: "When I was a child, I could run fast." (I was able to)
The context tells you which meaning is used.
When should you use must and can't?
Use must when you have evidence and you are making a logical conclusion.
- "You worked 12 hours today? You must be exhausted."
- "The lights are off. They must be out."
Use can't for the opposite — when the evidence tells you something is impossible.
- "You just ate a big lunch. You can't be hungry again!"
- "That can't be Tom. Tom is in Brazil this week."
Common mistake: For a negative strong guess, English speakers use can't, not "mustn't." "Mustn't" usually means something is not allowed ("You mustn't smoke here"). So:
- ✗ "He mustn't be at home." (sounds strange in this meaning)
- ✓ "He can't be at home." (I'm almost sure he isn't)
How do you talk about possibility in the past?
When you guess about something that already happened, use:
Modal + have + past participle
| Modal | Example | Meaning |
|---|---|---|
| must have | She must have left early. | I'm almost sure she left early. |
| might have / may have | I might have left my keys at work. | Maybe I left them there. |
| could have | You could have been hurt! | It was possible (but it didn't happen). |
| can't have / couldn't have | He can't have finished already. | I'm almost sure he didn't finish. |
In fast speech, native speakers shorten "have" to a soft sound: "must've," "might've," "could've." It sounds like "must-uv" or "might-uv."
Spelling warning: Many people write "could of" or "might of" because that is how it sounds. This is wrong. Always write could have or could've.
What do these modals sound like in real conversations?
Here are short dialogues you can practice out loud.
At work:
A: Where's Sarah? The meeting started ten minutes ago.
B: She might be stuck in traffic. The highway was closed this morning.
A: Or she may have forgotten. Let me send her a message.
At home:
A: Whose phone is buzzing?
B: It can't be mine. Mine is in my pocket.
A: Then it must be Dad's. He always leaves it on the couch.
Making plans:
A: Are you coming to the barbecue on Saturday?
B: I might. It depends on the weather. They said it could rain.
A: OK. Just text me if you decide.
Notice: In the last example, B answers only "I might." You can use the modal alone as a short answer when the verb is already clear. "Will you come?" — "I might." / "I may." / "Maybe."
Which mistakes do learners make most often?
1. Using "maybe" and "may be" as the same thing.
- "Maybe" (one word) is an adverb. It means "perhaps." → "Maybe he's sick."
- "May be" (two words) is modal + verb. → "He may be sick."
Both sentences mean the same thing, but the grammar is different. Don't write "He maybe sick."
2. Adding "to." ✗ "It might to rain." ✓ "It might rain."
3. Using "can" for uncertain possibility. "It can rain tomorrow" sounds odd. For a specific future guess, use might, may, or could: "It might rain tomorrow." Use "can" for general truths: "It can get very cold here in January." (This happens sometimes.)
4. Using "must" for a guess about the future. "Must" for possibility is about now or the past. For the future, say "should" or "will probably": "The bus should come soon."
5. Forgetting the past form. ✗ "She must forget my number yesterday." ✓ "She must have forgotten my number."
How can you practice this every day?
Use this simple 7-day plan. It takes about 10 minutes a day.
1. Day 1: Write 5 sentences with "might" about your plans this week.
2. Day 2: Look out the window. Make 3 guesses with "must" or "can't" about people you see. ("That man must be a runner. He's wearing running shoes.")
3. Day 3: Practice the past: write 5 sentences about something you lost or forgot, using "might have" or "must have."
4. Day 4: Read the dialogues above out loud. Record yourself and listen.
5. Day 5: Change all the "might" sentences from Day 1 into "may" and "could." Notice what feels natural.
6. Day 6: Watch a TV show or video in English. Every time you hear might, may, could, or must, pause and repeat the sentence.
7. Day 7: Talk to someone (a friend, a tutor, or even yourself) for 3 minutes about weekend plans. Use at least one modal of possibility in every answer.
Your next step
Right now, before you close this page, say three sentences out loud: one with might about something you could do tonight, one with must about someone you know, and one with might have about something you did yesterday. Then come back tomorrow and start Day 1 of the plan. Small, daily practice is what moves grammar from your notebook into your real conversations.
FAQ
What is the difference between might and may?
In everyday speech they mean almost the same. Might is more casual and common in conversation; may sounds a little more formal and appears often in emails and announcements.
Why can't I say 'he mustn't be at home'?
For a strong negative guess, English uses can't. Mustn't usually means something is not allowed, so 'He can't be at home' is the natural choice.
Is it 'could of' or 'could have'?
Always write could have or could've. 'Could of' only appears because could've sounds similar in fast speech, but it is a spelling mistake.