být — to be

Být ("to be") is the cornerstone verb of Czech: the copula that links subjects to descriptions, the existential verb for saying things are or exist, and the auxiliary that powers the past tense, the future, the conditional, and the passive. It is also the most irregular verb in the language. It is athematic (it has no thematic vowel joining stem to ending) and suppletive (different tenses are built from unrelated roots — js- in the present, byl- in the past, bud- in the future). This page is the complete reference. For a gentler walk-through, start with the introduction to být.

Present tense

The present forms jsem, jsi, jsme, jste are unstressed clitics: they cannot open a clause and must take second position (Dnes jsem doma). The third-singular negative is the irregular fused form není, not neje.

Past tense

The past is built from the l-participle byl / byla / bylo (sg.) and byli / byly / byla (pl.) plus the present-tense auxiliary jsem, jsi.... The participle agrees with the subject in gender and number; the auxiliary marks the person and is a clitic in second position. In the third person the auxiliary is dropped entirely — byl already means "he was."

The plural split follows animacy and gender: byli for masculine animate, byly for feminine and masculine inanimate, byla for neuter. Negation uses the ne- participle: nebyl jsem, nebyla...

Future tense

The future of být comes from the bud- root and is completely regular within itself. These forms are unique — no other Czech verb forms its future this way — and they do double duty as the imperfective future auxiliary: budu + an imperfective infinitive gives the future of any imperfective verb (budu pracovat, "I'll work").

For the auxiliary use in detail, see the imperfective future with budu.

Imperative

The imperative is built on the same bud- root.

Conditional

The conditional combines the l-participle (byl / byla...) with the special conditional auxiliary bych, bys, by, bychom, byste, by. This auxiliary is built on the same ancient root as být and is itself a second-position clitic. Note: in careful written Czech the 2nd-person singular is bys (and with reflexives it can contract, e.g. by ses), while colloquial speech often uses by jsi.

The same bych / bys / by... auxiliary builds the conditional of every Czech verb, which is why it is worth memorising here. See the present conditional with bych.

Case after být: nominative vs instrumental

As a copula, být can be followed by a predicate noun in two cases. The nominative states a plain identity. The instrumental frames it as a role, profession, or temporary capacity — and is the more idiomatic choice for jobs and functions.

See the instrumental as predicate for when to choose which.

Existential, locative, and impersonal uses

Je / jsou assert that something exists or is located somewhere — Czech's equivalent of English "there is / there are," but with no word for "there."

Být also drives impersonal and weather expressions, where there is no real subject and the verb sits in the neuter third singular.

Být as an auxiliary — summary

One verb supplies three different auxiliaries, which is the deepest reason být is non-negotiable:

• Past tense: present forms as auxiliary — byl jsem, dělal jsi.
• Future (imperfective): bud- forms + infinitive — *budu pracovat*.
• Conditional: bych / bys / by...
  • l-participle — dělal bych.

Common Mistakes

Key Takeaways

• Být is suppletive: present js- (jsem, jsi, je...), past byl-, future bud- (budu...).
• Present negatives are regular ne- except the irregular 3sg není.
• Past = byl/byla/bylo
  • clitic auxiliary; the participle agrees in gender/number, the 3rd-person auxiliary is dropped.
• Future budu... is unique and also serves as the imperfective future auxiliary (budu pracovat).
• Imperative buď, buďme, buďte; conditional uses byl bych with the bych/bys/by auxiliary.
• Predicate nouns take the nominative (identity) or, idiomatically for roles, the instrumental (Jsem studentem).