When do you refer to nouns by articles and when can/should you refer to them by their gender (ie, “er,” “sie,” “es”)? Why should or shouldn’t you in a given situation? Does referring to a noun by its gender affect the meaning or change anything about the sentence in any way?
Example:
Ich stehe auf dem Boden. Der ist kalt.
Ich stehe auf dem Boden. Er ist kalt.