Modals of Speculation: Could, Might, Must Be

Guessing about the present and past — how sure is sure?

Explanation

Modals of speculation help us talk about things we are not sure about. They tell the listener how confident we feel about a guess. At B2 level, you should be able to move smoothly between degrees of certainty in both the present and the past.

Present speculation

• must be — almost 100% sure. "She must be tired after that long flight."
• can't / cannot be — logically impossible. "He can't be 50 — he only looks 30."
• could be / might be / may be — possible, but you do not know. "She might be at the library, or she could be at home."

Past speculation

Add have + past participle after the modal:

• must have been — "They must have been delighted with the result."
• can't have been — "He can't have been serious when he said that."
• might have been / could have been — "She might have been working late last night."

Notice the word order: subject + modal + have + past participle. Never split "have" and the past participle.

Examples

Look at how the level of certainty changes the meaning of each sentence.

1. Must be: "There is a red Ferrari outside. The owner must be rich." (I am nearly certain.)
2. Might be: "There is a red Ferrari outside. The owner might be a doctor." (Just a possibility.)
3. Can't be: "He is still in his pyjamas. He can't be at the office yet." (Logically impossible.)
4. Must have been: "The streets are wet. It must have been raining last night."
5. Could have been: "You could have been hurt in that accident. Be careful!"
6. Might not have: "She might not have heard the news yet."

Common mistakes

• ❌ He must be tired yesterday. → ✅ He must have been tired yesterday. (For past time, use the modal + have + past participle.)
• ❌ She can be at work. → ✅ She could be at work. ("Can" is not used to speculate in English; use "could".)
• ❌ They must been late. → ✅ They must have been late. (The word "have" is essential.)
• ❌ Maybe he is tired. vs He must be tired. — "Maybe" is a separate adverb; modals of speculation are built into the verb phrase and sound more natural in formal writing and spoken English.
• ❌ He might be doctor. → ✅ He might be a doctor. (Do not forget the article before professions.)
• Confusing could have (past speculation or missed opportunity) with could (present ability). Context tells you which.

In real life

Dialogue: Two colleagues at the coffee machine on Monday morning.

Hana: “Late night? You look exhausted.”

Jules: “I must have been working until midnight. The report must be due today.”

Hana: “It is. And where is Daniel? He can't be late — he is the most punctual person I know.”

Jules: “He might be stuck in traffic. Or he could be at the dentist.”

Hana: “Well, whatever it is, he must have had a good reason.”

In just a few lines, the speakers mix almost certain, impossible, and possible guesses about both the present and the past — exactly the kind of natural, fluid guessing you will need in everyday conversation.