Hello, you will find this link helpful. If you need further help, don't hesitate to contact me. http://esl.fis.edu/grammar/rules/modal.htm

Should = used to indicate obligation, duty or correctness, typically when criticizing someone's actions. Could means past of can when it means that someone had the ability to do something or that something was possible. Can and could, like the other modal verbs, have developed quite arrange of meanings and uses.