(Here, Mark is in the nominative case because it's the subject of is, and businessman is in the nominative case because it's a subject complement; i.e., it renames the subject.)
It was I.
(Here, It is in the nominative case because it's the subject of was, and I is in the nominative case because it's a subject complement; i.e., it renames the subject.)
In modern English, a noun does not change its form in any of the cases (other than the possessive case). For example:
The woman saw the cat.
(Here, woman is in the nominative case because it's the subject of saw.)
The cat saw the woman.
(This time, woman is in the objective case, but there has been no change in spelling.)
A pronoun, however, does change its form depending on case. The nominative pronouns (or subjective pronouns as they're better known) are I, you, he, she, it, we, they, who, and whoever. Look at this example:
I saw the cat. The cat saw me.
(I is the subject of the verb saw. It is a subjective pronoun. However, it changes to me when it is not in the nominative case; i.e., when it's not the subject of a verb or a subject complement.)
