Uses

The auxiliary verb "could" is the past tense form of  "can". It is used to talk about:

1. Ability or possibility in past simple.
    Example: I could sleep very well last night.

2. Suggestion in present or future.
    Example: You could work in the afternoon.

3. Permission. Example:
    Could I use your computer?

4. Conditionals types 2 and 3.
    Examples: If I worked harder, I could make more money (type 2).
    If you had studied the topics, you could have passed the exam (type 3).