Your Roommate, a Marketing Major, Thinks That Debit Means Decrease

Your roommate, a marketing major, thinks that debit means decrease and credit means increase. And, that every account can be debited and credited and as result, every account can have both a debit and a credit balance. Explain to your roommate (1) the meaning of debit and credit; (2) which accounts can only be debited, which can only be credited, and which can be both debited and credited; and (3) which accounts normally have debit balances and which credit balances.