Company and Means

In Serbian, the instrumental case (инструментал) behaves differently depending on whether you are talking about an instrument or social company.

1. Means and Transport (Instrumental without prepositions)
When a noun functions as the tool or physical conveyance to accomplish an action, it appears directly in the instrumental case with no preposition:

• Пишем оловком. (I write with a pencil.)
• Путујемо возом. (We travel by train.)
• Сечем хлеб ножем. (I cut bread with a knife.)

Adding са or с here when denoting the pure instrument of an action (e.g., *пишем са оловком) is a frequent non-standard error; prototypical tools and means take the bare instrumental without prepositions. However, notice a subtle nuance: if an object represents an accompanying attribute or item carried rather than the direct operational tool, с / са can naturally appear (e.g., Борио се с пушком у руци [He fought with a rifle in his hand] vs. Борио се пушком [He fought with a rifle]).

Special exceptions where 'с / са' is used with means:
Standard authorities (such as Ivan Klajn) recognize two distinct categories where с / са is standardly permitted with means:

• Indeclinable words, symbols, and quoted expressions (accepted exception): When the means cannot inflect to show case endings—such as individual letters, non-inflected symbols, foreign abbreviations, or frozen quoted clauses—using с / са is fully standard and common: означити са X (to mark with X), дочекати са „Живели!“ (to greet with "Cheers!"). Alternatively, speakers can inflect a classifier noun or spelled-out word instead (e.g., означити словом X, означити иксом).
• Cardinal numbers, mathematical operations, and quantifiers (standard and permitted): In contemporary Serbian, cardinal numbers and quantity expressions routinely take с / са to accommodate the fact that numbers rarely inflect, including in arithmetic: помножити са пет (to multiply by five), посолити с мало соли (to salt with a little salt), закуцати са два ексера (to fasten with two nails), причврстити са три шрафа (to fasten with three screws). Note that the numerals два, две, три, четири have fully standard inflected instrumental forms (двама, двема, трима, четирима), which take the bare instrumental without prepositions (e.g., закуцати трима ексерима); alongside this, the modern construction keeping the numeral uninflected with с / са is widely accepted and common.

When a numeral declines normally, however, bare instrumental remains standard: the numeral један regularly inflects and takes the bare instrumental without prepositions (закуцати једним ексером).

2. Accompaniment and Association (с / са + Instrumental)
When someone or something accompanies the subject, you must use the preposition с (or its vocalized form са):

• Идем у биоскоп са сестром. (I am going to the cinema with my sister.)
• Разговарам с професором. (I am talking with the professor.)

This preposition is also used for associative additions, ingredients, or descriptive attributes:

• Кафа са шећером (coffee with sugar)
• Човек с брадом (a man with a beard)

3. Choosing between 'с' and 'са'
The basic preposition is с. Note that in standard Serbian orthography, с is never written with an apostrophe (do not write *с'). It expands to са for phonetic ease in specific environments:

• Before sibilants: с, з, ш, ж (e.g., са сестром, са зубаром, са школом, са женом). Note that affricates (ц, ч, џ, ћ, ђ) normally take с (e.g., с царем, с чашом).
• Before specific difficult initial consonant clusters (such as пс, кс, кћ, пч, пш, чк, e.g., са псом, са кћери, са пшеницом).
• With the first-person singular pronoun form: са мном (with me).

To explore specific noun case endings in detail, review singular instrumental forms and plural instrumental forms.