Expressing Ability : Can and Could

 Can

• Present ability: Use "can" to talk about what someone is able to do now.
  • "I can speak Spanish." (I have the ability to speak Spanish at this time.)
  • "She can play the piano." (She has the skill to play the piano now.)
  • "They can't come to the party." (They are not able to come to the party at this time.)

Could

• Past ability: Use "could" to talk about what someone was able to do in the past.
  • "I could run a mile when I was younger." (I had the ability to run a mile in the past, but maybe not now.)
  • "He could swim very well as a child." (He had the skill to swim well in the past.)
  • "They couldn't finish the project on time." (They were not able to finish the project in the past.)

Important Notes:

• "Could" for polite requests: "Could" is also used to make polite requests, but that's a different function than expressing ability. For example, "Could you please pass the salt?"
• "Be able to": If you need to use a different tense (like future or present perfect), you'll need to use "be able to" because "can" and "could" don't have those forms.
  • "I will be able to help you tomorrow." (Future)
  • "She has been able to play the guitar since she was ten." (Present perfect)

Examples to illustrate the difference:

• "I can ride a bike." (I have the ability to ride a bike now.)
• "I could ride a bike when I was five." (I had the ability to ride a bike in the past.)