The nominative case, as the name indicates, names or describes the subject of a sentence. The subject, noun or pronoun, performs the action in a sentence; for example: Pies głośno szczeka. (The dog barks loudly.) The subject here is pies in the nominative case. When you learn new nouns you will be able to see these in a dictionary, which will appear in the nominative case.
After być: When we name things, we often use the verb być (to be). The noun that follows that verb should take the nominative form. Examples:
To jest długopis. (This is a pen.)
To jest mój mąż. (This is my husband.)
To jest pies. (This is a dog.)
💡 If a noun, the subject of a sentence, becomes another noun and appears after jest (is), we use the instrumental case. Let’s see some examples:
Mój tata jest mechanikiem. (My dad is a mechanic.)
Jestem nauczycielką. (I am a teacher.)