Mastering English grammar can significantly improve your communication, and understanding modal verbs of probability is a key step. These powerful auxiliary verbs allow us to express how certain we are about a situation, whether in the present, past, or future. They're essential for conveying logical conclusions, possibilities, and uncertainties in a nuanced way.

This guide will break down the different modal verbs used for probability, providing clear explanations and examples to help you use them confidently. Let's dive into the fascinating world of expressing likelihood in English.

Modal verbs of probability are auxiliary verbs that help us express how certain or uncertain we are about something. They don't indicate ability, obligation, or permission in this context, but rather degrees of likelihood. We use them when we are not absolutely sure but want to express our logical conclusions or various levels of possibility.

Expressing Probability in the Present and Future

When talking about present or future situations, different modal verbs convey varying degrees of certainty. It's important to choose the right one to accurately reflect your level of conviction.

High Probability: Must and Can't

Must expresses a logical conclusion; something is logically probable. You are nearly certain, based on evidence.
Example: "He must be exhausted. He hasn’t slept for 24 hours!"
Example: "Aren’t they answering? They must be in bed."
Can't expresses logical improbability. You are nearly certain that something is not true or not happening.
Example: "Sue can’t have a ten-year-old daughter! She’s only 24!"
Example: "They can’t be out this late!"

Moderate Probability: May, Might, and Could

These modal verbs indicate that something is possible but not certain. They express a lower degree of probability than 'must' or 'can't'.

May/Might/Could all express probability in the present or future.
Example: "He might be lost. Or he may be stuck in traffic."
Example: "Dave and Beth aren’t at home. They could be at the concert, I suppose."
Example: "We may go to Greece for our holiday. We haven’t decided yet."
For negative statements, use may not or might not. Couldn't is rarely used in this context to express probability.
Example: "I might not be able to come tonight. I might have to work late."
Example: "They may not know where we are."

Using the Continuous Infinitive for Ongoing Probability

To talk about an action that is probably happening at the moment or will be ongoing in the future, we combine modal verbs with the continuous infinitive. The continuous infinitive is formed with be + -ing.

Must be + -ing (logically probable ongoing action)
Example: "You must be joking!"
Can't be + -ing (logically improbable ongoing action)
Example: "They can’t still be eating!"
Might be + -ing (possibly ongoing action)
Example: "Peter might be working late."
May be + -ing (possibly ongoing action)
Example: "They may be coming on a later train."

To express probability about events or situations that happened in the past, we use the perfect infinitive. This structure allows us to speculate about past actions or states based on current evidence or assumptions.

Using the Perfect Infinitive (Have + Past Participle)

The perfect infinitive is formed with have + past participle. This structure is used with modal verbs to express degrees of probability in the past.

Must have + past participle (logically probable past action/state)
Example: "He must have caught a later train."
Example: "He must have been exhausted."
Can't have + past participle (logically improbable past action/state)
Example: "She can’t have told him about us yet."
May have + past participle (possibly past action/state)
Example: "The letter may have got lost in the post."
Might have + past participle (possibly past action/state)
Example: "He might have changed his mind."
Could have + past participle (possibly past action/state)
Example: "They could have moved house."

Using the Continuous Perfect Infinitive for Ongoing Past Probability

To express probability about an action that was likely ongoing in the past, we use the continuous perfect infinitive. This is formed with have + been + -ing.

Must have been + -ing (logically probable ongoing action in the past)
Example: "She must have been joking."
Can't have been + -ing (logically improbable ongoing action in the past)
Example: "They can’t have been trying very hard."

While we use modal verbs to express probability in statements, question forms that directly start with Might she be…? are less common. Instead, we usually phrase these questions using Do you think…?.

Instead of: "Might she be married?"
We usually say: "Do you think she’s married?" (Answer: "She can’t be.")

If a modal verb is used in the question, it typically still follows Do you think…?.

Example: "Where do you think he’s from?" (Answer: "He might be Portuguese.")
Example: "Do you think he might be Portuguese?"

Understanding these structures for asking about possibilities is crucial for natural conversation.

Frequently Asked Questions About Probability Modals

Modal verbs of probability are auxiliary verbs (like must, can't, may, might, could) that express how certain or uncertain a speaker is about a situation. They convey logical conclusions or possibilities rather than ability or obligation.

How do I use 'must' and 'can't' for probability?

'Must' indicates a strong logical conclusion that something is probable (nearly certain). 'Can't' indicates a strong logical conclusion that something is improbable (nearly impossible). Both are based on evidence or reasoning.

When do I use 'may', 'might', and 'could' for probability?

'May', 'might', and 'could' all express a moderate level of possibility or uncertainty about a present or future event. They suggest something is possible, but you are not nearly certain. 'May not' and 'might not' are used for negative possibilities.

To express probability about past events, combine the modal verb with the perfect infinitive (have + past participle). For example, 'He must have caught the train' or 'She can't have told him'. For ongoing past actions, use the continuous perfect infinitive (have + been + -ing), like 'She must have been joking'.

Why are 'Do you think…?' questions more common with probability modals?

Direct questions like 'Might she be…?' are unusual. The construction 'Do you think…?' is more natural and common when asking about possibilities or probabilities in English. The modal verb, if used, often follows 'Do you think…?' as in 'Do you think he might be Portuguese?'

Expressing Probability in the Present and Future

When talking about present or future situations, different modal verbs convey varying degrees of certainty. It's important to choose the right one to accurately reflect your level of conviction.

High Probability: Must and Can't

Must expresses a logical conclusion; something is logically probable. You are nearly certain, based on evidence.
Example: "He must be exhausted. He hasn’t slept for 24 hours!"
Example: "Aren’t they answering? They must be in bed."
Can't expresses logical improbability. You are nearly certain that something is not true or not happening.
Example: "Sue can’t have a ten-year-old daughter! She’s only 24!"
Example: "They can’t be out this late!"

Moderate Probability: May, Might, and Could

These modal verbs indicate that something is possible but not certain. They express a lower degree of probability than 'must' or 'can't'.

May/Might/Could all express probability in the present or future.
Example: "He might be lost. Or he may be stuck in traffic."
Example: "Dave and Beth aren’t at home. They could be at the concert, I suppose."
Example: "We may go to Greece for our holiday. We haven’t decided yet."
For negative statements, use may not or might not. Couldn't is rarely used in this context to express probability.
Example: "I might not be able to come tonight. I might have to work late."
Example: "They may not know where we are."

Using the Continuous Infinitive for Ongoing Probability

Must be + -ing (logically probable ongoing action)
Example: "You must be joking!"
Can't be + -ing (logically improbable ongoing action)
Example: "They can’t still be eating!"
Might be + -ing (possibly ongoing action)
Example: "Peter might be working late."
May be + -ing (possibly ongoing action)
Example: "They may be coming on a later train."

Using the Perfect Infinitive (Have + Past Participle)

The perfect infinitive is formed with have + past participle. This structure is used with modal verbs to express degrees of probability in the past.

Must have + past participle (logically probable past action/state)
Example: "He must have caught a later train."
Example: "He must have been exhausted."
Can't have + past participle (logically improbable past action/state)
Example: "She can’t have told him about us yet."
May have + past participle (possibly past action/state)
Example: "The letter may have got lost in the post."
Might have + past participle (possibly past action/state)
Example: "He might have changed his mind."
Could have + past participle (possibly past action/state)
Example: "They could have moved house."

Using the Continuous Perfect Infinitive for Ongoing Past Probability

To express probability about an action that was likely ongoing in the past, we use the continuous perfect infinitive. This is formed with have + been + -ing.

Must have been + -ing (logically probable ongoing action in the past)
Example: "She must have been joking."
Can't have been + -ing (logically improbable ongoing action in the past)
Example: "They can’t have been trying very hard."

Instead of: "Might she be married?"
We usually say: "Do you think she’s married?" (Answer: "She can’t be.")

If a modal verb is used in the question, it typically still follows Do you think…?.

Example: "Where do you think he’s from?" (Answer: "He might be Portuguese.")
Example: "Do you think he might be Portuguese?"

Understanding these structures for asking about possibilities is crucial for natural conversation.

Modal Verbs of Probability

Expressing Probability in the Present and Future

High Probability: Must and Can't

Moderate Probability: May, Might, and Could

Using the Continuous Infinitive for Ongoing Probability

Using the Perfect Infinitive (Have + Past Participle)

Using the Continuous Perfect Infinitive for Ongoing Past Probability

Frequently Asked Questions About Probability Modals

How do I use 'must' and 'can't' for probability?

When do I use 'may', 'might', and 'could' for probability?

Why are 'Do you think…?' questions more common with probability modals?

Related topics

Modal Verbs of Probability

Expressing Probability in the Present and Future

High Probability: Must and Can't

Moderate Probability: May, Might, and Could

Using the Continuous Infinitive for Ongoing Probability

Using the Perfect Infinitive (Have + Past Participle)

Using the Continuous Perfect Infinitive for Ongoing Past Probability

Frequently Asked Questions About Probability Modals

How do I use 'must' and 'can't' for probability?

When do I use 'may', 'might', and 'could' for probability?

Why are 'Do you think…?' questions more common with probability modals?

Related topics

Modal Verbs of Probability

Understanding Modal Verbs of Probability

Expressing Probability in the Present and Future

High Probability: Must and Can't

Moderate Probability: May, Might, and Could

Using the Continuous Infinitive for Ongoing Probability