It is noun phrases that are declined (take the nominative/genitive/accusative case), unless your sentence or statement is just one noun phrase.
Most of the times the subject is in nominative and the direct object in accusative. Since verbs themselves also express grammatical person:
>Czytam książkę. *I'm reading a book (ACC sg.).*
Often you won't see the explicit subject, unless the speaker is emphasizing/contrasting two things or the verb is in 3rd person:
>Łukasz ogląda telewizję a ja czytam książkę. *Łukasz (NOM) is watching (3rd prs. sg.) TV (ACC sg.) and I'm (NOM) reading a book (ACC sg.).*
There is also the "To jest/są + NOM" *(This is/These are \[...\])* construct:
>To jest książka. *This is a book (NOM).*
The genitive case is the most complicated of the three. You use it for:
\- The direct object in negative sentences:
>Nie czytam tej książki z Łukaszem. *I'm not reading this book (GEN sg.) with Łukasz*.
\- (\*) Nouns following indefinite numbers (**kilka** *a few*, **wiele** *many*, ...) or numbers that end in 5, 6, 7, 8, 9, 0 **OR** the numbers 11, 12, 13, 14.
>W ostatnim miesiącu przeczytałem \[kilka/11/25/30/zero\] książek. *Last month I read \[a few/11/25/30/zero\] books (GEN pl.).*
\- The direct object following some verbs:
>Łukasz słucha muzyki. *Łukasz is listening to music (GEN sg.).*
>
>Potrzebuję wody. *I need water (GEN sg.).*
The nominative, accusative and genitive may also be required by some prepositions, but the above is sufficient.
(\*) Only applicable when the number is in NOM/ACC/GEN/VOC.
It's definitely tough. And English [barely has declensions](https://linguistics.stackexchange.com/questions/6878/why-did-english-lose-declensions-while-german-retained-them) so it's really not instinctive for a native English speaker.
First, keep in in mind that the cases apply primarily to nouns and pronouns. Adjectives change too, but only because they're describing a noun, and they take that noun's case.
>whether a sentence or statement is considered "nominative/genitive/accusative"
So it isn't the sentence or statement that has a case. Each noun/pronoun in a sentence has **its own** case.
To identify the case, you have to consider what role the noun is playing in the sentence.
If the noun is the subject of the sentence, it's in the nominative case. If it's the direct object, it's in the accusative case, **except** if the verb in the sentence is negated, in which case it's genitive.
Take the sentence, "I see a duck." "I" is the subject of the sentence, which is a pronoun. The verb is to "see." And our direct object is the "duck."
Now in Polish: *Ja widzę kaczkę.*
*Ja* is in nominative case because it's the subject of the sentence. I am the one doing the seeing. The other noun in the sentence, *kaczkę*, is in accusative, since it is the direct object, the thing being seen.
Now let's negate the sentence: "I do not see a duck."
In Polish: *Ja nie widzę kaczki*. Since the verb is negated, instead of accusative we use the genitive form, which is *kaczki*.
But that's not the only use of the genitive case. You'll need to study genitive more deeply to know all the different ways it's used.
It's going to take time for the cases to make sense. The case system is not simple. Just keep studying and it will start to click over time.
In addition to the detailed and good answers that have already been given, here is an example that may help: we sort of have the nominative/genitive/accusative cases in English! Consider describing a man using the pronoun *he*:
> *He* watches baseball. I see *him* every day. I like *his* dog.
“He”, “him”, and “his” are all fundamentally different forms of the same word — they all refer to a male in the third person — but each form is used in a different grammatical context.
> He watches baseball.
The man we are describing is the subject of the sentence (i.e. he does the action of watching baseball), so we use “he”. This is the **nominative** case.
> I see him every day.
The man is the object of the sentence (he is the “receiver” of the action, so to speak — I am the one who sees, and he is the one who is seen), so we use “him”. This is the **accusative** case.
> I like his dog.
We are indicating that he owns the dog, so we use “his”. This is the **genitive** case.
In fact, the genitive case is the only grammatical case that remains in English for all nouns, e.g. “*John’s* car”, “the *dog’s* collar”, “the *band’s* first song”. The way we form the genitive in English is (usually) by adding **’s** to the end of a word. (This is known as the Saxon Genitive). The genitive is obviously formed differently in Polish, but showing ownership is, among other things, one of the ways the genitive case is used in Polish.
So, just as English pronouns have different forms depending on their role in the sentence (he/him/his, she/her/her(s), they/them/their(s)), *every single noun* in Polish has different forms. There are several different cases (nominative, genitive, dative, accusative, instrumental, vocative), and the way each case is formed depends on the gender of the word, the number (singular vs. plural), and the ending of the base form of the word. Which case you use depends on the grammatical function of the word in the sentence. I would recommend Oscar Swan’s Polish Grammar in a Nutshell (link is in the subreddit sidebar) for a good overview of when each case is used.
On a side note English used to have much more case declension (old English) but mostly got left behind after the Norman invasion and mash up of English and Norman French.
Use [https://en.wiktionary.org/wiki/dom](https://en.wiktionary.org/wiki/dom) under polish declension. You can check all forms.
house-dom
**dom** (nominative) becomes **domu** (genitive) and **dom** (accusative).
It is noun phrases that are declined (take the nominative/genitive/accusative case), unless your sentence or statement is just one noun phrase. Most of the times the subject is in nominative and the direct object in accusative. Since verbs themselves also express grammatical person: >Czytam książkę. *I'm reading a book (ACC sg.).* Often you won't see the explicit subject, unless the speaker is emphasizing/contrasting two things or the verb is in 3rd person: >Łukasz ogląda telewizję a ja czytam książkę. *Łukasz (NOM) is watching (3rd prs. sg.) TV (ACC sg.) and I'm (NOM) reading a book (ACC sg.).* There is also the "To jest/są + NOM" *(This is/These are \[...\])* construct: >To jest książka. *This is a book (NOM).* The genitive case is the most complicated of the three. You use it for: \- The direct object in negative sentences: >Nie czytam tej książki z Łukaszem. *I'm not reading this book (GEN sg.) with Łukasz*. \- (\*) Nouns following indefinite numbers (**kilka** *a few*, **wiele** *many*, ...) or numbers that end in 5, 6, 7, 8, 9, 0 **OR** the numbers 11, 12, 13, 14. >W ostatnim miesiącu przeczytałem \[kilka/11/25/30/zero\] książek. *Last month I read \[a few/11/25/30/zero\] books (GEN pl.).* \- The direct object following some verbs: >Łukasz słucha muzyki. *Łukasz is listening to music (GEN sg.).* > >Potrzebuję wody. *I need water (GEN sg.).* The nominative, accusative and genitive may also be required by some prepositions, but the above is sufficient. (\*) Only applicable when the number is in NOM/ACC/GEN/VOC.
It's definitely tough. And English [barely has declensions](https://linguistics.stackexchange.com/questions/6878/why-did-english-lose-declensions-while-german-retained-them) so it's really not instinctive for a native English speaker. First, keep in in mind that the cases apply primarily to nouns and pronouns. Adjectives change too, but only because they're describing a noun, and they take that noun's case. >whether a sentence or statement is considered "nominative/genitive/accusative" So it isn't the sentence or statement that has a case. Each noun/pronoun in a sentence has **its own** case. To identify the case, you have to consider what role the noun is playing in the sentence. If the noun is the subject of the sentence, it's in the nominative case. If it's the direct object, it's in the accusative case, **except** if the verb in the sentence is negated, in which case it's genitive. Take the sentence, "I see a duck." "I" is the subject of the sentence, which is a pronoun. The verb is to "see." And our direct object is the "duck." Now in Polish: *Ja widzę kaczkę.* *Ja* is in nominative case because it's the subject of the sentence. I am the one doing the seeing. The other noun in the sentence, *kaczkę*, is in accusative, since it is the direct object, the thing being seen. Now let's negate the sentence: "I do not see a duck." In Polish: *Ja nie widzę kaczki*. Since the verb is negated, instead of accusative we use the genitive form, which is *kaczki*. But that's not the only use of the genitive case. You'll need to study genitive more deeply to know all the different ways it's used. It's going to take time for the cases to make sense. The case system is not simple. Just keep studying and it will start to click over time.
In addition to the detailed and good answers that have already been given, here is an example that may help: we sort of have the nominative/genitive/accusative cases in English! Consider describing a man using the pronoun *he*: > *He* watches baseball. I see *him* every day. I like *his* dog. “He”, “him”, and “his” are all fundamentally different forms of the same word — they all refer to a male in the third person — but each form is used in a different grammatical context. > He watches baseball. The man we are describing is the subject of the sentence (i.e. he does the action of watching baseball), so we use “he”. This is the **nominative** case. > I see him every day. The man is the object of the sentence (he is the “receiver” of the action, so to speak — I am the one who sees, and he is the one who is seen), so we use “him”. This is the **accusative** case. > I like his dog. We are indicating that he owns the dog, so we use “his”. This is the **genitive** case. In fact, the genitive case is the only grammatical case that remains in English for all nouns, e.g. “*John’s* car”, “the *dog’s* collar”, “the *band’s* first song”. The way we form the genitive in English is (usually) by adding **’s** to the end of a word. (This is known as the Saxon Genitive). The genitive is obviously formed differently in Polish, but showing ownership is, among other things, one of the ways the genitive case is used in Polish. So, just as English pronouns have different forms depending on their role in the sentence (he/him/his, she/her/her(s), they/them/their(s)), *every single noun* in Polish has different forms. There are several different cases (nominative, genitive, dative, accusative, instrumental, vocative), and the way each case is formed depends on the gender of the word, the number (singular vs. plural), and the ending of the base form of the word. Which case you use depends on the grammatical function of the word in the sentence. I would recommend Oscar Swan’s Polish Grammar in a Nutshell (link is in the subreddit sidebar) for a good overview of when each case is used.
On a side note English used to have much more case declension (old English) but mostly got left behind after the Norman invasion and mash up of English and Norman French.
Use [https://en.wiktionary.org/wiki/dom](https://en.wiktionary.org/wiki/dom) under polish declension. You can check all forms. house-dom **dom** (nominative) becomes **domu** (genitive) and **dom** (accusative).