COULD / SHOULD

Could states an option or a suggestion – other options to try.        

"I lost my wallet. What am I going to do?"                                  

"Well, you could . . . 

1. go back to the last place you saw it.
2. remember where you went today.
3. search your house and car.

Should states advice, a stronger suggestion - the best option to do.

"I lost my wallet. What am I going to do?"    

You should . . .

1. cancel your credit cards.
2. notify the campus police.

Suggestion – a helpful idea, plan, or possibility that someone mentions.
Advice     – an opinion you give someone about what they should do.

           COULD HAVE / SHOULD HAVE

Could have states past options, suggestions, or solutions that were not taken or done.  The person states them too late to act upon.

"My computer froze and I lost the project I was working on!"
 "You could have . . .

1. used auto-save
2. tried the escape key
3. used a back-up drive or site

Should have states advice about something in the past.  The person states them too late to act upon them.

"My computer froze and I lost the project I was working on!"
 "You should have . . .

1. used auto-save
2. tried the escape key
3. used a back-up drive or site