The nominative case (називни́й відмі́нок) is used to indicate the subject of a sentence - the person or thing performing the action. The word “називни́й” comes from the word “на́зва” (a name). Dictionaries use this case to list nouns. The only time you need to change the ending is when forming the plural.

When to use it?

1. When a noun is a subject of a sentence. The subject indicates who is doing the action.

Хло́пець чита́є кни́гу. - The boy reads a book.
Студе́нт йде в університе́т. - The student goes to university.
Співа́к співа́є пі́сню. - The singer sings a song.
Музика́нт гра́є на гіта́рі. - The musician plays a guitar.

2. Naming people, objects, etc.:

Його́ зва́ти Петро́. - His name is Petro.
Її зва́ти Марі́я. - Her name is Mariya.
Це мі́сто назива́ється Ки́їв. - This city is called Kyiv.

3. When a noun is used as an equivalent to the subject

Він — вчи́тель. - He is a teacher.
Мій улю́блений фрукт — я́блуко. - My favorite fruit is an apple.
Понеді́лок — пе́рший день ти́жня. - Monday is the first day of the week.

Note: Verb "to be" is usually omitted in the present tense, so we make a small pause when speaking or use a dash "—" when writing.