někdo / něco

Because někdo and něco are built directly on the pronouns kdo and co, their declension follows the exact same pattern. Note that někdo represents an animate entity, so its accusative matches its genitive (někoho). Něco represents an inanimate concept, so its accusative matches its nominative (něco).

Declension of Někdo (Someone):

• Nominative: někdo (someone)
• Genitive / Accusative: někoho (someone)
• Dative: někomu (to someone)
• Locative: někom (about someone)
• Instrumental: někým (with someone)

Declension of Něco (Something):
Notice the important spelling change! The base word co changes its c to č in all cases except the nominative and accusative. This shift is strictly preserved when the prefix ně- is added.

• Nominative / Accusative: něco (something)
• Genitive: něčeho (of something)
• Dative: něčemu (to something)
• Locative: něčem (about something)
• Instrumental: něčím (with something)

Note: The English translations above (like "to someone" or "about someone") are just helpful shortcuts. The required case depends entirely on the specific Czech verb or preposition being used, not the English translation (e.g., pomoci někomu = to help someone [Dative, no "to" in English]).

Important Usage Notes:
1. They are NOT indeclinable: A common misconception is that něco never changes form. While it remains něco in the nominative and accusative (e.g., zajímat se o něco), it must be fully declined in other cases (e.g., mluvit o něčem).
2. Translating "Anyone/Anything": In English, "any-" words are highly versatile. In Czech, use někdo/něco for simple unknown existence in questions (Je tam někdo? = Is anyone there?). However, in negative sentences, you must use negative pronouns instead (Nevidím nikoho = I don't see anyone). Additionally, in affirmative "free choice" contexts (like "Anybody can do it" or "You can choose anything"), Czech uses kdokoli(v) (anybody at all) and cokoli(v) (anything at all), rather than někdo/něco.
3. Adjectives with něco: When něco is in the nominative or accusative case, any following adjective must take the genitive case (e.g., něco dobrého = something good, něco nového = something new). However, in all other cases, the adjective simply agrees with the case of něco (e.g., o něčem zajímavém = about something interesting).
4. Past tense agreement: When used as a subject in the past tense, někdo always triggers masculine animate singular agreement (e.g., Někdo přišel = Someone arrived), while něco always triggers neuter singular agreement (e.g., Něco se stalo = Something happened).