być + Instrumental

When using the verb być (to be) to state someone's profession, nationality, or relationship, or to classify an object, animal, or concept, the noun phrase that follows must take Instrumental case endings. If you include an adjective, it must also match this case (-ym/-im for masculine/neuter, -ą for feminine, -ymi/-imi for plural; see Instrumental Adjective Agreement).

Basic Noun Endings:

• Masculine: Add -em to the consonant (e.g., student → studentem). If the noun ends in -k or -g, add -iem instead (e.g., Polak → Polakiem). Note: Some nouns drop a "mobile e" from their stem (e.g., pies → psem). For nouns ending in a soft consonant with an accent mark (-ć, -ś, -ń, -ź), change the accented letter to its unaccented counterpart and add -iem (e.g., koń → koniem). In clusters like -ść, only the final letter changes (e.g., gość → gościem).
• Neuter: Drop the final vowel -o and add -em (e.g., okno → oknem). If the stem ends in -k or -g, add -iem instead (e.g., jabłko → jabłkiem). Nouns ending in -e drop the -e and add -em (e.g., morze → morzem). Nouns ending in -ę take an expanded stem before adding the ending (e.g., imię → imieniem, zwierzę → zwierzęciem). Nouns ending in -um (like muzeum) do not change in the singular, but they decline regularly in the plural (e.g., muzeum → muzeami).
• Feminine: Change the final -a to -ą (e.g., kobieta → kobietą). For nouns ending in -i, add -ą (e.g., pani → panią, gospodyni → gospodynią). Feminine nouns ending in a historically soft consonant (like -c, -cz, -rz, -sz) add -ą (e.g., noc → nocą, rzecz → rzeczą), while those ending in a soft consonant add -ią (e.g., miłość → miłością, jesień → jesienią).
• Masculine in -a: Masculine nouns ending in -a follow the feminine rule and take -ą (e.g., artysta → artystą).
• Plural (all genders): Add -ami to the noun's stem, not the plural form (e.g., student + ami → studentami). Note: A few common words take an irregular -mi ending (e.g., ludzie → ludźmi, dzieci → dziećmi, bracia → braćmi, goście → gośćmi, konie → końmi, liście → liśćmi).

When NOT to use this pattern:
1. Standalone Adjectives: If you are only using an adjective to describe a state (like being tired or happy), it stays in the Nominative case. See Predicate Adjectives Stay Nominative.
2. Pointing things out: If you are introducing or pointing to someone or something using the word to (This is...), you use the Nominative case instead. Do not use the Instrumental case after To jest. For example, use To jest mój brat (Nominative) to point him out, but On jest moim bratem (Instrumental) to state his identity. See Identity Sentences With To.