Could

'Could' can be used to talk about the past, the present or the future.

'Could' is a past form of 'can'

• 
  When I was living in Boston, I could walk to work.
  
• 
  He phoned to say he couldn't come.
  
• 
  I could see him clearly but I couldn't hear him and then the videoconference line went dead.
  

'Could' is used to make polite requests. We can also use 'can' for these but 'could' is more polite.

• 
  Could you help me, please?
  
• 
  Could you lend me some money?
  
• 
  Could I have a lift?
  
• 
  Could I bother you for a moment?
  

If we use 'could' in
reply to these requests, it suggests that we do not really want to do
it. If you agree to the request, it is better to say 'can'

• 
  Of course I can.
  
• 
  I could help you if it's really necessary but I'm really busy right now.
  
• 
  I could lend you some money but I'd need it back tomorrow without fail.
  
• 
  I could give you a lift as far as Birmingham.
  

'Could' is used to talk about theoretical possibility and is similar in meaning to 'might'.

• 
  It could rain later. Take an umbrella.
  
• 
  He could be there by now.
  
• 
  Could he be any happier?
  
• 
  It could be Sarah's.