ни са ким / ни о чему

In Serbian, expressing ideas such as with no one or about nothing requires a special grammatical structure called tmesis (splitting).

1. The Splitting Rule:
As a general rule, when a negative pronoun or determiner combines with a preposition, the prefix ни- separates and is placed before the preposition:

• нико (no one) + са (with) $\rightarrow$ ни са ким (or ни с ким)
• нико (no one) + о (about) $\rightarrow$ ни о коме
• ништа (nothing) + о (about) $\rightarrow$ ни о чему
• ништа (nothing) + од (from) $\rightarrow$ ни од чега
• ништа (nothing) + за (for) $\rightarrow$ ни за шта

2. Parallel Formation With the и- Series:
The exact same splitting rule applies to the twin indefinite/polarity pronouns with the prefix и- (used in conditional clauses, questions, or with privatives):

• ико (anyone) + са $\rightarrow$ и са ким (with anyone)
• ишта (anything) + о $\rightarrow$ и о чему (about anything)
• икакав (any kind of) + у $\rightarrow$ и у каквом (in any kind of)

*Never write unsplit forms like *са иким or *о ичему.

3. Beyond Pronouns: Negative Determiners:
This splitting process applies not only to нико and ништа, but to all negative pronominal adjectives as well:

• никакав $\rightarrow$ ни у каквом (in no kind of)
• ничији $\rightarrow$ ни у чијем (in nobody's)
• ниједан $\rightarrow$ ни у једном (in no single one)

4. Strict Negative Concord:
Unlike in English, double (or triple) negation is mandatory in Serbian. Using a split negative phrase does not negate the sentence by itself; the predicate must still carry verb negation (such as не, нисам, or немам).

5. Special Contexts and Idioms:
Keep in mind these specific usages:

• The preposition без: To express simple absence (without anyone / without anything), Serbian does not use the negative ни- series; instead, standard Serbian pairs без with the non-negative indefinite series in и-: без икога (without anyone), без ичега (without anything), без икаквог (without any kind of). Note that phrases like ни без кога do occur grammatically, but only when ни acts as an emphatic or coordinating focus particle meaning neither / not even (e.g., Не могу ни без једног ни без другог — I can't do without either one; Не могу ни без кога од вас — I can't do without even one of you).
• Lexicalized noun usages: When ништа functions idiomatically as a regular neuter noun rather than a pronoun, the preposition stays in front without splitting: e.g., спао је на ништа (he came down to nothing) or створено из ничега (created out of nothing).

6. Spelling Note:
As a rule, these literal prepositional pronoun phrases are written as three separate words: ни са ким, ни о чему, ни за шта. However, standard orthography recognizes fused, one-word spellings when combinations have lexicalized into specialized adverbs, most notably низашта / низашто (meaning in vain, without reason, or for dirt cheap, distinct from literal ни за шта) and нипошто (meaning by no means, under no circumstances).