Could

Could is an important modal verb in English.  It is used for ability, possibility, and requests.
Could for Ability
"Could" is the past tense of "can."  These modals are used to
show ability.

Examples:

Present:        I can play guitar.
Past:              I could play guitar when I was younger.

As a negative, it is "could not", and is contracted as "couldn't".

Examples:

• I could not finish my work yesterday.
  
• I couldn't swim when I was a child.
  

Could for Requests
"Could" is used to make polite requests.

Examples:

• Could you please pass the salt?
  
• Could you turn down the music?

Could for PossibilityOften, "could" is used to express a degree of possibility.  For
example:

• It could rain today, but I am not sure.
  
• That could be Tom's brother.  He looks like Tom.
  

"Could" is also used in the 2nd conditional and 3rd conditional to show possibility and ability.

Examples:       

• If I had more money, I could buy a new car.
  

Or….I could buy a house; I could buy a boat, etc.

• If I were older I could drive. (Have the ability)