Modals of Possibility and Probability Lesson: Definition, Types, and Examples

Lesson Overview

• What Are Modals of Possibility and Probability?
• Expressing Strong Certainty (Logical Deductions in the Present)
• Expressing Possibility (Moderate or Weak Certainty in the Present/Future)
• Using "Maybe" and "Perhaps" (Adverbs of Possibility)
• Expressing Possibility and Probability in the Past

Modals of possibility and probability are modal verbs and related expressions that indicate how likely or unlikely something is. In other words, this grammar topic is essentially about the language we use for making guesses or deductions about situations. By mastering these modals, you can express degrees of certainty – from strong certainty to mere possibility – about present, future, or past events.

What Are Modals of Possibility and Probability?

Modals of possibility and probability are a subset of modal auxiliary verbs (like may, might, could, must, can't, etc.) used to express a speaker's assessment of the likelihood of a situation. They allow us to say that something is certain, probable, possible, or impossible based on evidence or opinion, rather than stating a fact. For example, compare these statements:

• Fact: "She is at home." (a definite statement – no modal)
  
  
• Possibility: "She might be at home." (using a modal to show we are not sure, just guessing)
  
  

In the second sentence, might shows that the speaker thinks it's possible (but not certain) that she is at home. This is what modals of possibility/probability do – they add a level of certainty or uncertainty to the statement.

Common Modals for Expressing Possibility and Probability

The most commonly used modals in this category are:

• Must (positive deduction) – indicates a strong likelihood or near-certainty that something is true.
  
  
• Can't / Cannot (negative deduction) – indicates a strong belief that something is not possible or not true.
  
  
• May, Might, Could – indicate that something is possible but not certain (often a medium or slight possibility).
  
  

These modals are used in the present/future context (using the base form of the verb) and can also be used to speculate about the past (using have + past participle). We will explore both. First, let's look at how to express different degrees of certainty about present situations.

Expressing Strong Certainty (Logical Deductions in the Present)

Sometimes we have evidence or logical reasoning that makes us almost certain about something, even if we don't know it as a fact. In such cases, we use must for positive statements and can't (cannot) for negative statements.

• Must (as a modal of probability) is used when we feel sure that something is true or consider it the only realistic explanation. It's like saying "I am sure this is true" based on logic or evidence.
  
  
  • Example: "Everyone is smiling and laughing – this must be a great party." (The speaker deduces that it's a great party from the evidence of people's behavior.)
    
    
• Can't (cannot) is used when we feel sure that something is not possible. It expresses a strong conviction that a situation or explanation is impossible.
  
  
  • Example: "He can't be serious – it's just a joke." (The speaker is almost certain that he is not serious.)
    
    

In the context of possibility/probability, "must" and "can't" are opposites:

• "Must" indicates we're nearly sure something is true (strong positive certainty).
  
  
• "Can't" indicates we're nearly sure something is not true (strong negative certainty, essentially saying "impossible").

Take This Quiz -

Modal Verbs Quiz Questions and Answers

Modal "Have to" as an Alternative for Strong Certainty

In informal American English, "have to" (or "have got to") can also be used to express a strong logical conclusion, similar to must. For example, "They have to be rich to own a house like that!" carries the same meaning as "They must be rich," indicating the speaker is nearly certain of their wealth. However, in formal grammar exercises, "must" is preferred for expressing deductions, since "have to" is not a modal verb but acts as a semi-modal. Keep this in mind if you encounter quiz questions asking for a modal – "must" will likely be the expected answer for strong certainty.

Certainly, Definitely: Adverbs of Strong Probability

Apart from modal verbs, English often uses adverbs to express certainty. Words like "certainly," "definitely," or "surely" can reinforce how sure we are. For example: "They have a huge mansion; that certainly means they're wealthy." Here certainly functions as an emphatic adverb, akin to saying "I am sure that means they're wealthy." In a quiz context, an answer might use "certainly" to convey strong probability (as in: "That certainly means they are very rich.") which emphasizes a high level of confidence in the statement.

• Certainly/Definitely – imply 90-100% certainty (almost sure).
  Example: "If he scored full marks, he definitely studied hard."
  
  
• Probably – implies a high probability but not absolute (perhaps ~70-90% sure).
  
  
  Example: "She's not here yet; she's probably stuck in traffic."

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Modal Verbs Quiz Questions: Exam!

Expressing Possibility (Moderate or Weak Certainty in the Present/Future)

When something is possible but we are not sure, we use the modals may, might, or could. These all fall into a middle range of certainty – neither certain nor impossible, just maybe true. According to grammar authorities, "might, may, or could" all convey that we think something is possible (in the present or future) but we're not sure. In meaning, they often overlap.

May / Might / Could (in affirmative statements): These modals are used interchangeably to indicate perhaps or maybe. For example:

• "It may rain later, so take an umbrella."
• "It might rain later…"
• "It could rain later…"
  
  

All three sentences above mean that rain is a possibility later (the speaker is not certain). Grammatically, they have the same basic meaning here.

Using "Maybe" and "Perhaps" (Adverbs of Possibility)

In addition to modal verbs, English often uses the adverbs maybe and perhaps to express possibility. "Maybe" and "perhaps" mean the same as "possibly" – indicating that something is not certain.


They are not modals (they are regular adverbs), but they are worth mentioning because our example quiz accepted "Perhaps" or "Maybe" as correct answers in a sentence requiring a slight possibility.

• Maybe is more common in everyday conversation.
  
  
• Perhaps means the same thing but is considered slightly more formal or literary (though it's also used in speech). According to the Britannica Dictionary, "perhaps" means ‘possibly but not certainly' and is a more formal word than "maybe.".
  
  

They are often used at the beginning of a sentence or before the subject:

• "Perhaps he missed the train." = Maybe he missed the train.
• "Maybe it will snow tonight."
  
  

Other Adverbs of Uncertainty

We already discussed certainly/definitely/probably for stronger confidence. In the middle of the spectrum, words like "possibly" and phrases like "perhaps maybe" (informal) can also emphasize the uncertainty:

• Possibly – explicitly states that something is possible, often used for slight possibility. e.g., "It's possibly a mistake." (This is essentially like saying "maybe it's a mistake.")
  
  
• Likely – not an adverb but an adjective/adverb that means probable. e.g., "She's likely (to) come late" (meaning she will probably come late).
  
  

These are not modals, but you might see them combined with modals: "It could possibly be true" (here could possibly is a very tentative statement – the double hedge shows it's a small possibility). Or "She will probably join us later." Using these adverbs with modals or main verbs adds nuance to the degree of certainty.

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Test your knowledge about Modals Verb quiz

Expressing Possibility and Probability in the Past

So far, we've dealt with present or general statements. But what about guessing about past events? English uses modal constructions with the present perfect infinitive (modal + have + past participle) to speculate about the past. This is crucial for questions or sentences like "My plants are dead. I can't have given them enough water."

When you see a modal followed by "have", it's signaling a deduction or possibility about a past situation. Here's how they work:

• Must have + [past participle] – indicates a strong probability that something happened in the past. The speaker is fairly sure. It's the past equivalent of present "must."
  
  
  • Example: "The ground is wet. It must have rained last night." (The speaker deduces that it rained last night – that's the logical explanation for the wet ground. They're not 100% certain, but it's the most likely reason)
    
    
  • Another: "She looks very happy this morning; she must have received good news." (Strong guess about a past cause.)
    
    
• May have / Might have / Could have + [past participle] – indicate that it's possible something happened in the past, but the speaker is not sure. This is a weak or moderate certainty (50% or less sure) about a past event.
  
  
  
  • Example: "I can't find my wallet. I might have left it at the office." (It's possible that I left it at the office – one possible explanation.)
  
  
  
• Can't have + [past participle] (or Couldn't have + [past participle]) – indicates a strong belief that something did NOT happen in the past. In other words, the speaker is almost sure that the past event didn't occur – it was impossible or didn't happen as imagined.
  
  
  
  • Example: "She can't have gone to the party - I saw her at home all evening." (It is basically impossible that she went to the party, since I have evidence against it.)
  
  
  
• May not have / Might not have + [past participle] – indicates it's possible something didn't happen in the past (the negative uncertainty). This is the past form of "maybe it did not…", about 50% or less certainty it didn't happen.
  
  
  • Example: "He might not have received your email yet." (Maybe he didn't receive it – we're not sure.)
    
    
  • Example: "I may not have locked the door; I'll go check." (It's possible I didn't lock it.)
    
    

Using this knowledge, you should be able to confidently decide which modal or expression fits a given sentence that asks you to express a certain degree of possibility or probability. Always consider the strength of certainty implied by the modal.