It marks the subject - the person or thing doing the action.
Answers kto? (who?) or co? (what?)
This is the dictionary form - the shape you'll see in every wordlist.
Also used after to jest / to są (this is / these are).
Anna czyta książkę.
Anna is reading a book.
To jest kot.
This is a cat.
To są studenci.
These are students.
When to use it
Three easy signals tell you the Nominative is coming.
The subject of any sentence: Piotr pracuje.
After to jest / to są: To jest moja siostra.
In lists and labels: menus, name tags, signs.
After być + a profession (jestem lekarzem), you use Instrumental instead. That's a separate case - covered later.
Ten samochód jest nowy.
This car is new.
To są moi rodzice.
These are my parents.
Singular: no change
In the singular, the Nominative is the dictionary form. Nothing changes.
Group
Ending
Example
Masculine
kot, dom, samochód
Kot śpi.
Feminine
kobieta, kawa, książka
Kawa jest gorąca.
Neuter
okno, dziecko, mleko
Dziecko śpi.
You can spot gender from the ending: -a is usually feminine, -o/-e/-ę neuter, most consonant endings masculine.
Ten dom jest duży.
This house is big.
Plural: the big split
Polish has TWO plural forms, depending on who's in the group.
Oni plural: any group with at least one man (masculine personal).
One plural: everything else - women only, children, animals, objects.
This choice affects the endings on nouns, adjectives, and pronouns.
Oni są w domu.
They are at home. (men or mixed)
One są w domu.
They are at home. (women / things)
'One' plural endings
The easy plural: things, women, animals.
Group
Ending
Example
Masc. non-personal
-y / -i
kot → koty, samochód → samochody
Feminine
-y / -i
kobieta → kobiety, książka → książki
Neuter
-a
okno → okna, mieszkanie → mieszkania
After k, g use -i instead of -y: książka → książki, noga → nogi.
Te książki są nowe.
These books are new.
Moje koty śpią.
My cats are sleeping.
'Oni' plural endings
Groups with men - endings soften and consonants often change.
Group
Ending
Example
Basic ending
-i / -y
student → studenci
Prestigious
-owie
szef → szefowie, pan → panowie
Soft -e
-e
lekarz → lekarze, nauczyciel → nauczyciele
See Zawody and Narodowości for the full patterns - this is the trickiest area of Polish grammar.
Nasi studenci są bardzo mądrzy.
Our students are very smart.
Ci panowie to lekarze.
These men are doctors.
Watch out: after 'to jest' → always Nominative
Even in negation, even with adjectives, this construction never changes case.
To jest kot. - This is a cat. (Nominative)
To nie jest kot. - This is not a cat. (still Nominative!)
To jest moja siostra. - This is my sister. (Nominative)
Compare with On jest lekarzem (He is a doctor) - here być takes Instrumental. But To jest lekarz uses Nominative. The little word to makes the difference.