When should one use could instead of can?

Could is the past tense of can. Today I can go to the football game. Yesterday, I could not go to the football game. Could is also the present tense of the subjunctive mood of can. If I could do it (present tense of subjunctive mood), I would do it (present tense of conditional mood of the verb do). Could is also the present tense of the conditional mood of can. If I had enough money, I could buy the guitar.