Could is used in two ways in English:

1) Past ability:

–I can speak English now. I couldn't when I was a child.

2) Polite questions:

–Could I borrow your car?
–Could you speak slower?
–Could we check out later?

Note that could is the simple past form of the verb can. The negative form of it is couldn't (could not).

Examples of Using Could

• You could also take I-75 north through Georgia into Tennessee.
• Fill it up with regular, please. Could you also check the oil and the tires?