can (could) and shall (should) are modal verbs and go together with another verb to indicate the tense
"I could fail math class next week."
I could have failed the math exam last week without all the tutoring you gave me."
"should' : you need to review in your dictionary all the uses of this word quite apart from that as past tense of 'shall'
'could' : Again, you will find these uses described in your dictionary.