s + Instrumental

The preposition s (or its vocalized form se) is used to express accompaniment, association, or interaction. When expressing accompaniment or association, it requires the noun, pronoun, or adjective to be in the instrumental case.

1. When to use s / se
Use it when people or things are together:

• káva s mlékem (coffee with milk)
• mluvit s bratrem (to speak with a brother)

2. Vocalization (s vs. se) and Pronunciation
To make pronunciation easier, s changes to se before:

• Words starting with sibilants (s, z, š, ž): se sestrou (with a sister)
• Difficult consonant clusters: se psem (with a dog)
• The pronoun mnou: se mnou (with me)

Pronunciation note: Before voiced consonants (like b, d, g), the preposition s is pronounced as a [z]. For example, s bratrem is pronounced as [z bratrem]. Note that before the consonant v (and other sonorants), the preposition s can be pronounced as either [s] or [z] (e.g., s vodou can be pronounced as [s vodou] or [z vodou]).

3. Third-Person Pronouns
When using third-person pronouns after a preposition like s, they gain an initial n-. For example, you must say s ním (with him/it), s ní (with her), and s nimi (with them) instead of using their base forms.

4. The "s sebou" Exception
When the reflexive pronoun sebou means "to take along with oneself," it is an exception to the vocalization rule and is written s sebou (e.g., vzít si jídlo s sebou - to take food with you). However, se sebou is still used when doing something with oneself in a non-carrying sense (e.g., mluvit se sebou - to talk to oneself, být spokojený sám se sebou - to be satisfied with oneself). Additionally, the bare instrumental sebou (without any preposition) is used for body movements (e.g., házet sebou - to thrash about).

5. Fixed Verbal Phrases
Many common Czech verbs require s + instrumental as a fixed phrase, even when English uses a different preposition. Examples include počítat s něčím (to count on something), souhlasit s (to agree with), and seznámit se s (to get acquainted with).

6. The "With" Trap
In English, "with" is used for both accompaniment ("with a friend") and tools ("with a pen"). In Czech, these are strictly separated. Use s + instrumental for accompaniment, but use the instrumental of means (the bare instrumental case with NO preposition) for tools or methods.

7. Spoken Czech Note
In casual, everyday spoken Czech, you will frequently hear the plural instrumental ending leveled to -ma (e.g., s kamarádama instead of the standard s kamarády). See Standard instrumental plural versus Common Czech -ama for more details.

8. Note on s + Genitive
While this pattern focuses on the instrumental case, be aware that the preposition s / se can also be used with the genitive case to mean "down off a surface" (e.g., vzít knihu se stolu - to take a book off the table). In modern Czech, this is often replaced by z / ze + genitive.