Can, Could, May, and Might: Common Mistakes and How to Use Them Correctly

Introduction

Can, could, may, and might are among the most commonly confused modal verbs in English. Although they look similar, each one has a different meaning and purpose. In this guide, you’ll learn the differences, avoid common mistakes, and use them confidently in everyday English.

Have you ever paused while writing an email and wondered whether you should write:

• Can I review this document?
• Could I review this document?

Or perhaps you’ve asked yourself whether may and might mean exactly the same thing.

If so, you’re not alone.

These four small words cause confusion for many English learners because they often seem interchangeable. However, each one has its own purpose, and choosing the right word can make your English sound more natural, polite, and professional.

Over the years, I’ve noticed that many people understand the grammar rules but still struggle to choose the right modal verb in real-life situations. The good news is that once you understand the basic differences, using them becomes much easier.

 Free Printable Worksheet

Practise as you learn! Download the FREE Worksheet (PDF) on Can, Could, May, and Might and complete the exercises while reading this lesson.

In this guide, you’ll learn:

• when to use can and could
• when to use may and might
• how they affect the tone of your sentences
• common mistakes to avoid
• plenty of everyday examples

Let’s get started.

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What Are Modal Verbs?

Can, could, may, and might are modal verbs.

A modal verb works together with a main verb to express ideas such as:

• ability
• permission
• possibility
• requests
• probability

Unlike ordinary verbs, modal verbs never describe the main action themselves. Instead, they change the meaning or tone of the main verb.

For example:

✅ She can speak English.

✅ We might finish early.

Auxiliary Function & Grammar Characteristics

Modal auxiliary verbs act as specialized “helper” words that modify the main verb’s mood, attitude, or likelihood. Standard English verbs take different suffixes based on tense and subject (run, runs, running, ran). Modal verbs, however, belong to a closed class of defective verbs—meaning they lack infinitive forms (no “to can”), participle forms (no “canning” or “canned”), and third-person singular endings (no “cans”).

Pedagogical Perspective / ESL Teaching Tip

From an ESL instructional standpoint, understanding modal verbs is essential because they bridges pure grammar with social pragmatism. Using can instead of could when speaking to a senior manager doesn’t just change sentence structure; it subtly shifts the level of respect and social boundary expressed in professional communication.

For formal academic writing guidelines regarding modal usage, refer to the Merriam-Webster Dictionary standards.

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The Golden Rule of Modal Verbs

Modal verbs follow one very important rule.

They never change their form, regardless of the subject.

Correct:

• I can swim.
• He can swim.
• They can swim.

Not:

❌ He cans swim.

The verb after a modal must always be in its base form.

Correct:

• She can write.
• We could finish today.

Incorrect:

❌ She can writes.

❌ We could to finish.

Remember this rule and you’ll avoid many common grammar mistakes.

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1. Can vs. Could

Although could is the past form of can, it has several additional uses.

A. Ability

Can = Present Ability

Use can to describe something you are able to do now.

Examples:

• I can read blueprints without my glasses.
• She can speak three languages.
• We can finish the report today.

Could = Past Ability

Use could for a general ability in the past.

Examples:

• When I was younger, I could stay awake all night studying.
• She could run very fast as a child.
• My grandfather could repair almost anything.

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B. Requests and Permission

This is where tone becomes important.

Can = Casual

Use can with friends, family, classmates, or colleagues.

Examples:

• Can I borrow your pen?
• Can you send me that file?
• Can we start now?

These sound friendly and natural.

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Could = More Polite

Use could when speaking to customers, clients, managers, teachers, or someone you don’t know well.

Examples:

• Could you please send me the progress report?
• Could I speak with the manager?
• Could you explain that again?

Using could makes your request sound more respectful.

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C. Could for Possibility

We also use could to talk about possible situations.

Examples:

• The delivery could be delayed because of heavy rain.
• This solution could save us time.
• The meeting could finish earlier than expected.

Here, could suggests a possible outcome, not a certainty.

Could for Hypothetical / Conditional Situations

We also use could in conditional sentences (especially second conditionals) to express imaginary or unreal scenarios in the present or future.

• Example: “If we had a larger budget, we could hire another web developer.”
• Example: “I could travel the world if I didn’t have to work this summer.”

Past Ability vs. Specific Achievement (“Could” vs. “Was Able To”)

A classic exam trap involves describing past ability. Use could for general past abilities (“When I was young, I could run fast”). However, when talking about succeeding in a single, specific difficult action in the past, standard English prefers was/were able to or managed to rather than could.

• ❌ Incorrect: “The road was flooded, but we could reach the destination.”
• ✔️ Correct: “The road was flooded, but we were able to reach the destination.”

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2. May vs. Might

Both may and might express possibility.

The main difference is that may usually suggests a stronger possibility, while might often suggests a more uncertain one.

However, in modern English, they are frequently interchangeable.

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A. May = A Real Possibility

Use may when something seems reasonably likely.

Examples:

• It may rain this afternoon.
• We may launch the new website next week.
• The manager may approve your proposal.

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B. Might = A Less Certain Possibility

Use might when you’re less certain or simply making a guess.

Examples:

• We might expand the project next year.
• I might go for a walk later.
• She might join us if she finishes work early.

Although might often sounds less certain than may, the difference is usually quite small in everyday conversation.

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C. May for Formal Permission

May is traditionally used when asking or giving permission in formal situations.

Examples:

• May I come in?
• May I review the project plans?
• You may leave once the meeting has finished.

Today, many native speakers simply say Can I…? in everyday conversation, but May I…? still sounds more formal and polite.

Might Have vs. May Have (Past Possibility)

When discussing actions that possibly took place in the past, we combine the modal with have + past participle (May have / Might have).

• May Have: Used when the past outcome is still unknown or open.
  • Example: “The package may have arrived at the office yesterday.” (We still need to check if it arrived).
• Might Have: Used when speculating about past events, especially counterfactual situations that did not happen.
  • Example: “You might have hurt yourself!” (You didn’t get hurt, but the danger existed).

Modal Verbs in Negative Sentences

Negating modal verbs is straightforward: place not directly after the modal verb, followed by the base verb. Notice the spelling variation for cannot:

• Cannot / Can’t: Expresses inability or total prohibition (“You cannot park here”).
• Could not / Couldn’t: Expresses past inability or impossibility (“He couldn’t open the locked file”).
• May not: Expresses formal refusal of permission or real negative possibility (“You may not enter without a badge”).
• Might not: Expresses negative possibility with higher uncertainty (“She might not attend the webinar today”).

Section: Degrees of Certainty Spectrum

To visualize how these four modals function when predicting future events, place them on a probability spectrum:

• 100% Certain: Will $\rightarrow$ “The site will launch tomorrow.”
• ~70% Likely: May $\rightarrow$ “The site may launch tomorrow.”
• ~50% Possible: Could $\rightarrow$ “The site could launch tomorrow.”
• ~30% Uncertain: Might $\rightarrow$ “The site might launch tomorrow.”

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Common Mistake: Maybe vs. May Be

Many learners confuse these two expressions.

Maybe (one word)

Means perhaps.

Examples:

• Maybe I’ll go tomorrow.
• Maybe she’s busy.

May be (two words)

Is a modal verb followed by the verb be.

Examples:

• The contractor may be delayed.
• The meeting may be cancelled.

A simple trick:

If you can replace it with perhaps, use maybe.

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Quick Comparison Table

Situation	Best Choice	Example
Present ability	Can	I can update the website today.
Past ability	Could	I could swim when I was five.
Casual request	Can	Can you help me?
Polite request	Could	Could you explain this?
Formal permission	May	May I leave early?
Likely possibility	May	It may rain later.
Less certain possibility	Might	We might visit next week.

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Frequently Asked Questions

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🎁 Free Printable Worksheet

Enjoyed this lesson on Can, Could, May, and Might? Reinforce what you’ve learned with our FREE printable worksheet.

✅ Printable PDF
✅ Detailed answer key with explanations
✅ Perfect for self-study or classroom use

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Practice Exercise: Test Your Modal Verb Skills

Choose the correct modal verb for each sentence:

1. __________ you please speak a little louder? I can’t hear you. (Can / Could)
2. She __________ swim when she was only four years old. (can / could)
3. You __________ leave the examination room once your paper is collected. (may / might)
4. We __________ go to the beach tomorrow, but it depends on the weather forecast. (might / can)
5. I have checked the records, and the error __________ be fixed by updating the plugin. (may / might)

Answer Key & Explanations:

1. Could — Polite request addressed to someone in a formal or respectful setting.
2. could — General past ability (when she was four years old).
3. may — Formal grant of permission in an official testing context.
4. might — Indicates an uncertain possibility dependent on an external condition (weather).
5. may — Indicates a strong, real possibility based on verified factual evidence.

If you want more practice with auxiliary structures, check out our full lesson on English tenses and verb forms.

Final Thoughts

Although can, could, may, and might are only four small words, they play a big role in everyday English. They help you express ability, ask permission politely, make requests, and talk about possibilities with confidence.

The best way to master them is through regular reading, listening, and practice. As you notice these modal verbs in books, emails, and conversations, ask yourself why the speaker or writer chose that particular word.

With time, choosing the right modal verb will become second nature—and your English will sound more natural, accurate, and confident.

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