It's important to note that "could" has two meanings:
* Past ability: "I could swim when I was younger." (meaning: I was able to swim in the past)
* Past possibility: "I could have gone to the party, but I was tired." (meaning: it was possible for me to go, but I didn't)
While "could" is the past tense of "can," it can also be used in the present tense to express possibility or polite requests.