оба/обе

The Russian words for 'both' are more than just numbers; they are specific pointers to a pair. Unlike the simple numeral два/две, оба/обе implies 'both of the...' rather than just 'two.'

1. Nominative and Inanimate Accusative
When the pair is the subject or an inanimate direct object, the noun following the numeral must be in the Genitive Singular. Neuter nouns follow the masculine pattern (оба):

• Оба брата (Both brothers - Masculine)
• Оба окна (Both windows - Neuter)
• Вижу оба стола (I see both tables - Inanimate Accusative)
• Обе сестры (Both sisters - Feminine)

2. Animate Accusative
If the pair consists of people or animals and is the direct object, the construction follows the animate plural accusative rule. The noun takes the Genitive Plural form, and the numeral takes its Genitive form (which uses plural adjective endings):

• Вижу обоих братьев (I see both brothers)
• Вижу обеих сестёр (I see both sisters)

3. Mixed Groups and Defaults
The masculine form оба is the default for any pair that is not exclusively two feminine nouns. However, when referring to mixed groups (like a brother and a sister, or a table and a bed), оба can only be used as a standalone pronoun (e.g., Брат и сестра — оба здесь). It cannot be used as a direct modifier before two conjoined nouns of different genders (e.g., saying оба брат и сестра is ungrammatical).

4. Use with Personal Pronouns
These numerals are frequently used with personal pronouns to say 'both of us,' 'both of you,' etc. They follow the same gender and case rules: мы оба (both of us, mixed/masculine), вы обе (both of you, feminine).

5. Declension (Oblique Cases)
When the case changes (Genitive, Dative, Instrumental, Prepositional), the gender distinction is preserved in the stem, and the noun is always in the plural:

• Masculine/Neuter Stem (обо-): обоих (Gen), обоим (Dat), обоими (Instr), об обоих (Prep).
• Feminine Stem (обе-): обеих (Gen), обеим (Dat), обеими (Instr), об обеих (Prep).

Warning: A very common mistake, even among native speakers, is using the masculine обо- stem for feminine nouns in oblique cases. Always maintain the обе- stem for feminine nouns (e.g., never say с обоих сторон; the correct form is с обеих сторон).

6. Stress Shift
A crucial detail for pronunciation is the stress shift. In the Nominative and Inanimate Accusative, the stress falls on the first syllable. In all oblique cases, the stress shifts to the stem vowel.

7. Pluralia Tantum Restriction
You cannot use оба/обе with nouns that only exist in the plural (like ножницы - scissors). To express 'both' with these nouns, you must use the phrase и те и другие (ножницы) or обе пары (ножниц).

8. Adjective Agreement
When an adjective comes between the numeral and the noun in the nominative/inanimate accusative, it takes the Genitive Plural for masculine and neuter nouns (оба новых стола), and the Nominative Plural for feminine nouns (обе новые сёстры).

9. Substantivized Adjectives
Nouns that decline like adjectives (substantivized adjectives like животное, ванная, больной) do not take the Genitive Singular. Instead, they take plural forms after оба/обе: Genitive Plural for masculine and neuter (оба животных, оба больных) and Nominative Plural for feminine (обе ванные).

10. Word Order and False Friends
The numeral usually precedes the noun. Note that the masculine/neuter genitive/accusative form обоих sounds similar to the noun обои (wallpaper), but they are completely unrelated.