Quick answer: Can expresses present ability (I can swim), permission (You can leave now), or possibility. Could is the past form of can, but it also expresses polite requests (Could you help me?), past ability (She could run fast as a child), and theoretical possibility (That could be true). For a fuller definition, see the Cambridge Dictionary entry for ‘can’.
Comparison Table
| Word | Type | Main Uses | Example |
|---|---|---|---|
| can | modal verb | present ability, permission, possibility | I can drive. Can I go? |
| could | modal verb | past ability, polite requests, possibility/hypothesis | She could dance. Could you help? |
Using Can
Can is a modal verb used for three main purposes: (1) present ability — what someone is able to do right now; (2) permission — asking or giving permission informally; (3) possibility — something that is theoretically possible in general.
She can speak three languages.
Can I use your phone?
Too much sun can damage your skin.
You can park here after 6pm.
Can in Questions and Negatives
The negative of can is can’t (or cannot in formal writing). In questions, can comes before the subject:
I can’t find my keys.
Can you hear me?
He cannot attend the meeting.
Can they join us tomorrow?
Using Could
Could has several distinct uses: (1) past ability — what someone was able to do in the past; (2) polite requests — more formal and indirect than can; (3) theoretical possibility — something that might be true; (4) conditional — what would be possible if something were true.
When I was young, I could run very fast. (past ability)
Could you pass the salt, please? (polite request)
That noise could be the wind. (possibility)
With more practice, you could become excellent. (conditional)
Could Have + Past Participle
Could have + past participle expresses a possibility in the past that did not happen, or that we are uncertain about:
She could have won, but she made an error.
That could have been very dangerous.
I could have sworn I left my bag here.
Common Mistakes
✗ When I was five, I can read already.
✓ When I was five, I could read already.
Note: for past ability, always use could, not can.
✗ Can you send me the report by Friday? (too informal in a business email)
✓ Could you send me the report by Friday?
Note: could is more polite and formal for requests; use can only in informal contexts.
✗ The firefighters could save everyone in the fire. (implies past ability, not a specific event)
✓ The firefighters were able to save everyone in the fire.
Note: for a specific successful achievement in the past, use was/were able to, not could.
Also worth knowing: May vs Might. Browse all confusing word pairs →
Quick Check: Can or Could?
Four questions — choose the correct word for each sentence.
1. “When she was young, she ___ climb any tree.”
2. “___ you open the window, please?” (polite request)
3. “I ___ speak French fluently.” (present ability)
4. “That ___ be the answer — we’re not sure.”
Practise Modal Verbs with Free Exercises
Test your knowledge of can, could, and other modal verbs with interactive exercises.
Start Practising →Frequently Asked Questions
What is the difference between can and could?
Can expresses present ability, permission, or possibility: “I can drive.” Could is the past form of can and also expresses polite requests, past ability, and theoretical possibility: “Could you help?” / “She could sing beautifully as a child.”
When do I use could instead of can?
Use could for: (1) past ability: “I could swim at age 4”; (2) polite requests: “Could you help me?”; (3) theoretical/uncertain possibility: “That could be true”; (4) conditionals: “You could do it if you tried.”
Is “could you” more polite than “can you”?
Yes. Could you is more formal and polite. Can you is fine in informal speech with friends. In professional emails or when speaking to strangers, could you is preferred.
Can I use can for future ability?
Yes. Can is used for ability in the present and near future: “I can see you on Monday.” Will be able to is used for more distant or specific future ability: “After the course, I will be able to drive.”
What is the negative of could?
Couldn’t (could not): “I couldn’t understand what he said.” In formal writing, write it as two words: “could not.”
What is “could have done” used for?
Could have + past participle has two uses: (1) a past possibility that did not happen: “She could have passed, but she gave up”; (2) mild criticism of a past action: “You could have told me!”
Can can express possibility?
Yes: “Too much stress can cause illness” (general possibility). Could expresses a weaker, more uncertain possibility: “That could be true” (= maybe, but I’m not certain).
Is “can I” or “may I” more correct for permission?
May I is more formal and traditionally “correct” for asking permission. Can I is very common in everyday speech. Could I is a polite middle ground.
What is the difference between could and was able to?
For past ability in general: both work: “He could/was able to read at age 3.” For a specific successful achievement in the past, use was/were able to (not could): “We were able to catch the last train.”
How do can and could appear in IELTS speaking?
In IELTS speaking, can is used for present abilities and factual possibilities. Could is used for polite answers (“Could I expand on that?”) and hypotheticals (“Could be an issue”). Overusing can for everything sounds informal; mixing them correctly demonstrates grammar range.