било ко / било шта / било који

The pattern било ко, било шта, and било који allows speakers to express absolute arbitrariness across nominal and adjectival slots.

1. Origin, Morphology, and Extensions
The particle било originates historically from the neuter singular active past participle (радни глаголски придев / l-participle) of the verb бити (literally been, historically used in elliptical concessive clauses such as [нека би] било). In contemporary Serbian, it has grammaticalized into an autonomous, invariable particle written as a separate word.

While било remains unchanged, the pronoun or pronominal adjective inflects regularly according to standard declension patterns:

• било ко (nominative) → било кога (genitive/accusative) → било коме (dative/locative) → било ким / било киме (instrumental). Standard normative usage strongly prefers the full forms кога and коме for the pronoun ко (especially in careful writing), though shortened forms like ког and ком are also encountered as colloquial or stylistic variants.
• било шта (nominative/accusative) → било чега (genitive) → било чему (dative/locative) → било чим / било чиме (instrumental)
• било који (masculine), било која (feminine), било које (neuter) follow standard adjectival declension across all cases. Note the masculine accusative singular distinction: animate direct objects take the genitive form (било којег(а) / било ког(а)), while inanimate objects match the nominative (било који). Masculine/neuter genitive forms are било којег(а) / било ког(а), and dative/locative forms are било којем(у) / било ком(е).

The particle било also freely extends to other interrogative-relative pronominals and adverbs, such as било какав (any kind of), било чији (anyone's), and било где (anywhere).

2. Prepositional Word Order
Unlike indefinite and negative pronouns with the bound prefixes ни- (нико, ништа) and и- (ико, ишта)—which strictly demand preposition splitting by tmesis (ни с ким, и с ким; never *са никим, *са иким)—the word било is an independent particle. In standard Serbian, prepositional placement follows distinct stylistic registers:

• Particle-first (codified standard norm): било с ким, било за шта, било у ком граду / било у којем граду. Standard normative authorities (such as Ivan Klajn) recommend this order as the primary choice for standard, formal, and careful writing.
• Preposition-first (informal and colloquial): са било ким, за било шта, у било ком граду / у било којем граду. While frequent in spontaneous everyday speech, it is traditionally regarded in normative grammar as colloquial.
• Postpositive (literary and traditional): с ким било, за шта било, у ком граду било. Normative authorities recognize this word order, which is encountered chiefly in literature and established idiomatic phrases.

3. Syntactic Constraint: Prohibition Under Verbal Negation
A crucial syntactic distinction between Serbian and English is that било ко and било шта cannot be used under standard sentential verbal negation. In English, 'anyone' is used with a negated verb (I didn't see anyone). In standard Serbian, using било кога here (*Нисам видео било кога) is ungrammatical. Standard Serbian requires strict negative concord with the нико / ништа series whenever the verb is negated:

• Correct: Нисам видео никога. (Not: *Нисам видео било кога.)
• Correct: Нисмо купили ништа. (Not: *Нисмо купили било шта.)
• Affirmative free choice: Било ко може да уђе. ("Anyone at all may enter.")

4. Semantic Contrasts
Learners frequently confuse било- with other indefinite series:

• било ко vs. неко: Неко refers to an unspecified, particular entity (someone), whereas било ко means any person whatsoever without distinction (anyone).
• било ко vs. ико: Ико is a negative polarity item restricted to questions, conditional clauses, and comparisons (Да ли је ико дошао?). In affirmative free-choice statements (Anyone can join), standard Serbian requires било ко, not *ико.
• било који vs. сваки: Сваки is a distributive universal quantifier (every single one), whereas било који signals an arbitrary, interchangeable selection (whichever / any one).