can, could

Using can, cannot (can't), could, and could not (couldn't) in English

1. Ability in the Present

Can expresses what someone is able to do now.

• Example: “I can swim.” (I have this ability now.)
• Negative: “I cannot swim.” / “I can’t swim.”

2. Ability in the Past

Could expresses a general ability in the past.

• Example: “I could swim when I was five.” (I had this ability in the past.)
• Negative: “I could not swim when I was five.” / “I couldn’t swim…”

3. Possibility

Can describes something that is generally possible; could describes something that is possible but less certain or more hypothetical.

• “It can get very hot here in summer.” (general possibility)
• “It could rain later.” (less certain possibility)

4. Permission

Can is commonly used to ask for and give permission; could is more polite or formal.

• “Can I go to the restroom?”
• “Could I borrow your pen?” (more polite)

5. Requests

Can and could can both be used to make requests. Could is softer and more polite.

• “Can you help me?” (direct)
• “Could you help me?” (polite)

6. Offers

• “I can help you with that.” (offer in the present)

7. Suggestions (with could)

• “We could try a different restaurant.” (a possible suggestion)

8. Impossibility

The negative forms show that something is not possible.

• “He can’t be serious.” (It seems impossible he is serious.)
• “That couldn’t be true.” (I strongly believe it is impossible.)

9. Form

• can → negative: cannot / can’t
• could → negative: could not / couldn’t
• These modal verbs do not change form. They are followed by the base form of the verb:
  • “She can drive.” (NOT “can drives” or “can to drive”)
  • “He could help.”