jeden / jedna / jedno

The numeral jeden declines exactly like the hard demonstrative pronoun ten, ta, to (using the root jedn- in most cases). It must agree with the noun in gender, case, and number.

1. Basic Forms (Nominative)

• Masculine: jeden (e.g., jeden muž, jeden hrad)
• Feminine: jedna (e.g., jedna žena)
• Neuter: jedno (e.g., jedno auto)

2. Declension
While it modifies nouns like an adjective, it follows a pronominal declension pattern and changes in different cases. For example, in the accusative, you say Mám jednu sestru (I have one sister) or Vidím jednoho psa (I see one dog). Note that for masculine inanimate nouns, the accusative remains identical to the nominative: Vidím jeden hrad (I see one castle). After prepositions, it also declines: s jedním kamarádem (with one friend).

3. The "Indefinite Article" Nuance
While Czech doesn't have articles, it frequently uses jeden to mean "a certain" or "a particular." You use it when introducing a specific new entity into a conversation or story (e.g., Byl jednou jeden král - Once upon a time there was a king).
Potkal jsem jednoho chlapa. (I met a [certain] guy.)
Do not confuse this with nějaký, which means "some [random/unspecified]" guy.

4. Plural Forms
Normally, "one" is singular. However, for nouns that only exist in the plural (pluralia tantum), or for regular nouns that come in standard pairs or sets, you must use the plural forms jedni, jedny, or the neuter plural jedna (e.g., jedny brýle - one pair of glasses, jedny boty - one pair of shoes, jedna vrata - one gate). Additionally, plural forms are used as a correlative pronoun meaning "some [people/things]" when contrasted with "others" (druzí), for example: Jedni souhlasili, druzí ne. (Some agreed, others didn't).

5. Other Uses of Jeden

• Compound Numbers: In numbers like 21 or 31, the two-word form dvacet jedna can either agree with the noun (dvacet jedna koruna) or take the genitive plural (dvacet jedna korun). When using the nominative singular noun, the verb must be singular. If using the genitive plural, the verb must be neuter singular. However, the one-word inverted form jednadvacet acts as a single block and always takes the genitive plural (jednadvacet korun) and a neuter singular verb (jednadvacet korun zbylo).
• Jeden z (One of): To say "one of," use jeden followed by the preposition z (or ze) and the genitive plural: jeden z mých přátel (one of my friends).
• Ani jeden (Not a single one / Neither): To express "not a single one," use ani jeden. Because Czech uses negative concord, this must be paired with a negated verb: Neviděl jsem ani jednoho. (I didn't see a single one.)
• Nejeden (Many a / More than one): The negative form nejeden means "many a" or "more than one" and declines exactly the same way as jeden: To už se stalo nejednomu člověku. (That has happened to many a person.)
• Reciprocal (Each other): The phrase jeden druhého (and its declined forms) is used to express "each other." For example: Pomáhají jeden druhému. (They help each other.)
• Correlative: Used in pairs to mean "one... the other" (jeden... druhý). For example: Jeden mluvil a druhý poslouchal. (One spoke and the other listened.)
• Generic Subject: It can act as a generic pronoun meaning "one" or "you" in general statements: Jeden nikdy neví. (One never knows.)
• Time expressions: Jeden is heavily used in time, specifically in the accusative: v jednu hodinu or just v jednu (at one o'clock).
• Only / Alone: It is frequently used to mean "only" or "alone." For example: Byl na to sám jeden. (He was all alone for it.)