Must, Might, or Can’t Have? English Modal Verbs of Deduction Explained

You see a friend’s coat on the chair and say, “She must be home.” Your phone rings, you don’t recognise the number, and you think, “It might be a scam call.” Your keys aren’t in your bag, so you mutter, “I can’t have left them at work… or could I?”

That’s modal verbs of deduction: the grammar English speakers use to make educated guesses out loud. It’s one of those structures that barely gets a mention in textbooks, yet native speakers use it constantly — in gossip, in detective shows, in “where did I put my phone” moments. Once you have it, you’ll notice it everywhere, and you’ll sound noticeably more natural.

Why This Trips People Up

Most learners meet “must,” “might,” “could,” and “can’t” as separate, unrelated topics: “must” for obligation, “might” for future possibility, “could” for ability or permission. Deduction is a different job entirely. Here, these words aren’t about rules or permission — they’re about how confident you are in a guess, based on evidence you already have.

The good news: there are really only three levels of certainty to learn, and the same three modals cover both the present and the past (with one small change in form).

The Three Levels of Certainty

Must — “I’m almost certain this is true”

Use must when the evidence strongly points one way and you’d be surprised to be wrong.

“The lights are on and the car’s outside — they must be home.”
“You’ve been awake since 4am. You must be exhausted.”

Might / May / Could — “This is possible, but I’m not sure”

These three are interchangeable here and simply mean “maybe.” Use them when you genuinely don’t know.

“She’s not answering. She might be in a meeting.”
“That noise could be the neighbours, or it could be the pipes again.”

Can’t — “I’m almost certain this is NOT true”

This is the one that catches people out, because the logical-sounding negative of “must” would be “mustn’t.” English doesn’t work that way. For strong certainty that something is false, you need can’t.

“That can’t be right — he left an hour ago.” (not “mustn’t be right”)
“She can’t be twenty. She looks about fifteen.”

Keep “mustn’t” for obligation (“You mustn’t be late”) and reach for “can’t” every time you mean “I’m sure this is false.”

Talking About the Past: Add “Have”

Everything above works the same way for the past — you just swap the base verb for have + past participle. The levels of certainty don’t change, only the timeframe.

Must have: “The grass is wet. It must have rained last night.”
Might/could have: “I’m not sure why he’s upset. Someone might have said something.”
Can’t have: “She can’t have finished already — I only gave her the task five minutes ago.”

A reliable pattern to remember: no “to,” no -s, and no “did.” It’s always modal + base verb in the present, and modal + have + past participle in the past — never “must to be,” “musts be,” or “did must.”

The Mistake Spanish Speakers Make Most Often

In Spanish, deber de covers a lot of this ground (“debe de estar en casa”), which tempts learners to translate “must” too literally and overuse it for every guess — strong or weak. The fix is to listen for your own confidence level before you speak. If you’re really just speculating, reach for “might” or “could,” and save “must” for when the evidence is solid. Overusing “must” makes you sound more certain — and sometimes more dramatic — than you mean to be.

The second common slip is reaching for “mustn’t” in the negative. If you remember just one rule from this article, make it this one: “can’t” is the negative of certainty, not “mustn’t.”

Quick Self-Test

Fill the gap with must, might/could, or can’t (adding “have” where the situation is in the past):

1. The streetlights are off and the shops are closed. It ___ be the middle of the night.
2. I don’t recognise this jacket. It ___ be my brother’s — I honestly don’t know.
3. He said he was in London yesterday, but I saw a photo of him in Paris. He ___ been in two places at once.
4. You look freezing. You ___ have forgotten your coat.

(Answers: 1. must; 2. might/could; 3. can’t have; 4. must have)

Make It Automatic

Reading the rule is the easy part — the real skill is reaching for the right modal in the half-second you have during a real conversation. That only comes from using it out loud, ideally with someone who’ll gently correct you when “must” slips out instead of “might.” If you want that kind of focused, low-pressure practice, a one-on-one lesson with one of Parlazo’s native English teachers is a natural place to drill exactly this, and you can always start with a free level test if you’re not sure where to pick up.