s / se + Instrumental

The preposition s/se is one of the most common words in Czech, but it has a very specific job: it means "with" when talking about company, addition, interaction, or characteristics.

1. Accompaniment, Interaction, and Characteristics
Use s/se when you do something alongside someone (s kamarádem - with a friend), when things go together (káva s mlékem - coffee with milk), for interaction (mluvit s bratrem - to speak with a brother), or to describe features (muž s brýlemi - a man with glasses). To form the instrumental singular, masculine nouns (except those ending in -a or -í) and hard neuter nouns take -em (soft neuters take -em, -etem, or -ím if ending in -í). Note that masculine nouns ending in -a (like kolega or táta) take the ending -ou, and those ending in -í (like Jiří or průvodčí) decline like adjectives and take -ím. Feminine nouns take -ou (hard) or -í (soft). Adjectives take -ým/-ím (masculine/neuter) and -ou/-í (feminine). In the plural, masculine and neuter nouns usually take -y/-i (some neuters add -y to the plural stem -at-, or take -ími), while feminine nouns take -ami/-emi/-mi. Plural adjectives take -ými/-ími. In everyday spoken Czech, you will frequently hear the universal plural ending -ma (e.g., s dobrýma kamarádama). However, for paired body parts (ruce, nohy, oči, uši), the ending -ma (e.g., rukama, nohama, očima, ušima) is the mandatory standard form even in formal written Czech. Crucially, any adjectives or pronouns modifying these nouns must also take the dual ending -ýma/-íma (e.g., s velkýma očima, s dlouhýma nohama). Note also the highly frequent irregular plurals for 'people' (lidé) and 'children' (děti), which take the -mi ending: s lidmi (with people) and s dětmi (with children).

2. The "Tool" Exception
Do not use s/se for tools, instruments, or means of transport! If you write "with a pen" or travel "by car", Czech uses the instrumental of means without any preposition (píšu perem, jedu autem). Using s here would absurdly imply you are traveling accompanied by a car.

3. When to use "se" and Pronunciation
The base form is s. It changes to se before words starting with s, z, š, ž (se sestrou - with a sister) or difficult consonant clusters (se psem - with a dog). Note that s always becomes se before mnou (se mnou - with me). A highly practical phrase is s sebou (carrying with oneself), often used for takeaway (kávu s sebou - coffee to go). Note that s sebou is pronounced without the initial 's' sound, simply as [sebou]. It is used for carrying or taking things along, while the vocalized form se sebou is used for interaction or states (e.g., mluvit sám se sebou - to talk to oneself). Finally, the bare word sebou (without 's') is used for body movements (e.g., házet sebou - to thrash about) or in the phrase samo sebou (of course).

Pronunciation note: Due to voicing assimilation, the preposition s is pronounced as [z] before voiced consonants (like b, d, v, h; e.g., s bratrem sounds like [z bratrem]). This can be tricky because the preposition z/ze (from) sounds like [s] before voiceless consonants. You must rely on context and the noun's case to distinguish 'with' from 'from'.

4. Personal Pronouns
When using s/se with personal pronouns, you must use their instrumental forms: se mnou (with me), s tebou (with you), s ním (with him/it), s ní (with her), s námi (with us), s vámi (with you all/formal), and s nimi (with them).

5. Verbs Requiring s + Instrumental
Certain common verbs strictly require the preposition s followed by the instrumental case. Core examples include souhlasit s (to agree with), setkat se s (to meet with), and rozloučit se s (to say goodbye to).

6. Don't confuse it with other words
Be careful not to confuse the preposition se with the reflexive pronoun se used in inherent reflexive verbs with se (like smát se). Also, don't confuse it with the preposition z/ze, which means "from" and takes the genitive case.