два / три / четыре + Genitive Singular

In Russian, the numbers 2, 3, and 4 (and 1.5) follow a unique rule. Historically, the number 2 inherited this syntax from an ancient 'dual' number system, and the numbers 3 and 4 adopted it by analogy. When these numbers are in the Nominative (or Inanimate Accusative) case, the noun following them is in the Genitive Singular.

1. Gender Agreement:
Only the number 'two', the word for 'both', and 'one and a half' change based on the gender of the noun:

• два / оба / полтора: Masculine and Neuter (два стола - two tables, полтора часа - an hour and a half)
• две / обе / полторы: Feminine (две книги - two books, полторы недели - a week and a half)

три and четыре remain the same for all genders.

2. Adjective Agreement (After the Number):
If you add an adjective after the number, it must be plural:

• Masculine/Neuter: Adjective is Genitive Plural (два больших стола - two big tables).
• Feminine: Adjective is usually Nominative Plural (две большие книги - two big books), though Genitive Plural is also grammatically acceptable.

3. Adjective Agreement (Before the Number):
When the phrase is in the Nominative or Inanimate Accusative, adjectives placed before the number take the Nominative Plural regardless of gender (e.g., эти два стола - these two tables, последние три года - the last three years), with some exceptions like 'целых'. However, if the phrase shifts to the Animate Accusative, the preceding adjective must also shift to the Animate Accusative Plural (e.g., я вижу этих двух мальчиков - I see these two boys).

4. The Animate Accusative Rule:
For simple numbers (2, 3, 4), if the noun is animate, both the numeral and noun shift to the Animate Plural Accusative. Crucially, this means the noun changes from Genitive Singular to Genitive Plural (which serves as the Animate Accusative Plural). For example: я вижу двух мальчиков (I see two boys). This also applies to feminine nouns, where the numeral loses its gendered distinction: я вижу двух девочек (I see two girls). Note that 'полтора/полторы' does not undergo the animate accusative shift; it ignores animacy and remains in its nominative form even with animate nouns (e.g., я вижу полтора землекопа - I see one and a half diggers).

5. Compound Numbers and Animacy:
Crucially, compound numbers ending in 2, 3, or 4 (like 22, 33, 44) do not undergo the animate accusative shift. Their Accusative form is always identical to the Nominative: я вижу двадцать два мальчика (I see twenty-two boys). Note that numbers 11-19 (like 12 - двенадцать, 13 - тринадцать, and 14 - четырнадцать) are not compound numbers; they end in the suffix -надцать and naturally take the Genitive Plural following the 5-20 rule.

6. Stress Shifts:
Some nouns shift their stress when used in this 'paucal' count form compared to the standard Genitive Singular. For example, 'час' (hour) is около часа (around an hour - standard Genitive) but два часа (two hours - count form with stress on the end).

7. Oblique Cases:
This Genitive Singular rule only applies to the Nominative and Accusative cases. In oblique cases (Genitive, Dative, Instrumental, Prepositional), the numeral and the noun simply agree in case, and the noun is always in the Plural (e.g., около двух столов - around two tables, с тремя мальчиками - with three boys).

8. Substantivized Adjectives:
Nouns that are formed from adjectives (like животное - animal, учёный - scientist, мороженое - ice cream) take the Genitive Plural after 2, 3, and 4, not the Genitive Singular (e.g., два животных - two animals, три учёных - three scientists).

9. Plural-Only Nouns:
Nouns that only exist in the plural form (pluralia tantum, like часы - watch/clock, очки - glasses, деньги - money) cannot be used with standard два, три, or четыре. Instead, they require collective numerals (двое, трое, четверо) (e.g., двое часов - two watches). Note that compound numbers ending in 2, 3, or 4 cannot be combined with plural-only nouns at all (e.g., 'двадцать два суток' and 'двадцать двое суток' are both strictly ungrammatical). You must use a lexical workaround, such as двадцать два дня (twenty-two days).