How to Express Certainty, Probability, and Possibility in English

Before you learn how to express certainty, probability, or possibility in English, it’s important to understand the foundation first: modal verbs.

What are Modal Verbs?

Modal verbs are auxiliary verbs used to express different “modes” or conditions such as possibility, ability, necessity, or prohibition. Common examples include must, can, may, might, should, could, and ought to.

In this lesson, you’ll discover how to use modal verbs to express certainty, probability, and possibility clearly and confidently. These skills are essential for both everyday conversations and professional communication.

1. Expressing Certainty in English

Certainty means being completely confident or free from doubt. In English, we often use the modal verbs must, can’t, and couldn’t to show certainty in the present.

✅ Examples (Present):

• You must go to the barangay health center today for your vaccination.

• I can’t go to school today because I have a headache.

• Jenny must finish her assignments this evening.

To express certainty about the past, we use must have, can’t have, and couldn’t have.

✅ Examples (Past):

• I must have eaten before I went to school this morning.

• He can’t have driven in Beverly Hills without a permit.

• Jeff couldn’t have told me this morning because I wasn’t there.

💡 Quick Tip: Use “must” when you are sure something is true, and “can’t” or “couldn’t” when you are sure it’s impossible.

2. Expressing Probability in English

Probability refers to the likelihood that something will happen. Modal verbs like should and ought to express events that are likely but not certain.

✅ Examples (Present/Future):

• My brother should go to the doctor for treatment.

• I ought to buy bananas every day as a potassium source.

• One hour ought to be enough for this task.

For the past, we use should have or ought to have.

✅ Examples (Past):

• I should have called Jenny sooner.

• It rained yesterday; I should have taken an umbrella.

• They ought to have built a waiting shed near the school.

💡 Quick Tip: “Should” and “ought to” often suggest advice or logical expectation.

3. Expressing Possibility in English

Possibility means something might happen, even if it’s not certain. The most common modals for possibility are could, may, and might.

✅ Examples (Present/Future):

• I could buy fruit when I go to the supermarket.

• I may go now.

• I might see you on Wednesday or Thursday.

• I might visit the dentist tomorrow for a braces adjustment.

💡 Quick Tip: Use “may” and “might” in formal contexts. “Could” is often used for polite suggestions or potential actions.

4. Formal vs. Informal Usage

When writing formal papers, reports, or academic essays, avoid contractions like shouldn’t or couldn’t. While they are natural in spoken English, formal writing prefers the full forms: should not, could not, might not.

By mastering how to express certainty, probability, and possibility, you gain the ability to communicate ideas more clearly—whether you’re talking about the past, present, or future. Modal verbs not only make your English sound more natural but also help you express confidence, advice, or doubt politely.

Remember: must = strong certainty, should = probability, may/might/could = possibility.

With practice, you’ll be able to choose the right words effortlessly, making you a more effective and confident communicator.