The nominative case is the base form of the noun and signals that we are talking about the subject of the sentence. You can locate the subject easily because it's the person / thing that performs the action and what tells you how to conjugate the verb.
The questions for the nominative case are "Wer?" (Who?) and "Was?" (What?)
The nominative case is also used after the verbs "sein," "werden," and "bleiben." (Warning: In this case it is NOT the subject!)
The subject is always in the nominative case. NO EXCEPTIONS!
The subject is the person/thing that performs the action. It also tells you how to conjugate the verb.
The verbs sein, werden, and bleiben use a subject complement. That means the verb is followed by a noun in the nominative case (even though it’s not the subject).
The subject complement is not the subject, but it does represent the subject in a certain sense. That's why it is in the nominative case.
Tip: Where is the subject?
The subject is often the first word of the sentence, but it can also come in the middle.
In English, we always goes subject-verb-object. In German, on the other hand, the different nouns can go in any order. This isn't done just to be silly - it's done to emphasize something else in the sentence. Whatever comes first in the sentence is what the speaker wants to emphasize.