Genitive after quantities

1. When to use it:

• Quantifiers: hodně (a lot), málo (a little/few), trochu (a little), několik (several), pár (a couple).
• Measures, Containers & Fractions: kilo (kilo), litr (liter), sklenice (glass), kousek (piece), and fractions like třetina (third) and čtvrt (quarter) are declinable nouns that govern a partitive genitive. The fraction půl (half) acts as an indeclinable numeral, but also forces the following noun into the genitive (e.g., půl hodiny).
• Numbers 5+: Numbers from five upwards require the genitive plural, with the exception of compound numbers ending in 1–4 (see Section 5). (Remember that numbers 1–4 agree with the noun's case, and gender for 1 and 2, rather than taking the genitive).

2. Singular vs. Plural & Adjectives:
Use the genitive singular for mass/uncountable nouns (e.g., trochu vody - a little water). Use the genitive plural for countable nouns after quantifiers and numbers 5+ (e.g., hodně aut - a lot of cars). However, fractions take the genitive singular of countable nouns when referring to a part of a single item (e.g., půl hodiny - half an hour, čtvrt jablka - a quarter of an apple). Any adjectives modifying the counted noun must also take the genitive case (e.g., pět dobrých lidí). Note: The word rok (year) has a highly frequent irregular genitive plural form let after numbers 5+ and quantifiers (e.g., pět let, kolik let). Similarly, lidí is the irregular genitive plural of člověk (person).

3. The "Nominative/Accusative Only" Rule:
For numerals 5+ and quantifiers, the genitive rule only applies when the phrase is the subject or direct object. In any oblique case (Dative, Locative, Instrumental), the rule "switches off" regardless of whether a preposition is present: declinable numerals (like pět) and the noun take the required case (e.g., Pomáhám pěti lidem - Dative without a preposition; s pěti lidmi - Instrumental with a preposition). Some quantifiers (like několik, mnoho, kolik) also decline in oblique cases (e.g., s několika lidmi), while indeclinable quantifiers (like pár, hodně) do not change, but the noun still takes the required case (e.g., s pár lidmi). Note that in strict formal contexts, hodně is often replaced by the declined quantifier mnoho in oblique cases (e.g., s mnoha lidmi). Additionally, in oblique cases, trochu typically changes to the declinable noun trocha (e.g., s trochou štěstí), and because it acts as a measure noun, the following noun remains in the genitive. Nouns following Measures and Containers always remain in the genitive (e.g., se sklenicí vody). However, nouns following the fraction půl decline into the required case in oblique cases, while the fraction remains unchanged (e.g., před půl rokem - Instrumental, po půl roce - Locative). The noun čtvrt can also remain frozen in certain time or measurement phrases (e.g., před čtvrt rokem).

4. Verb Agreement:
When a quantity phrase is the subject of a sentence, the verb must be in the third-person singular neuter. In the past tense, this means the verb will always end in -o (e.g., Přišlo pět lidí. - Five people came). See Section 5 for an exception regarding compound numbers.

5. Compound Numbers:
Compound numbers (21, 22, etc.) have two forms. If written as a single inverted word (jednadvacet), the whole phrase acts like a quantifier and forces the genitive plural (e.g., jednadvacet knih). If written as separate words (e.g., dvacet jedna), the noun traditionally agrees with the final digit (e.g., dvacet jedna kniha, dvacet dvě knihy). In this case, the verb also agrees with the noun instead of taking the neuter singular (e.g., ležela tam dvacet jedna kniha, ležely tam dvacet dvě knihy). However, in modern colloquial Czech, even these separate forms frequently take the genitive plural (e.g., dvacet jedna korun).