Местоимения
Russian Pronouns
Why do pronouns change form?

Russian is a case language — the ending of a word signals its grammatical role in the sentence. Pronouns are no exception. English actually does this too: "I / me / my / mine" and "he / him / his" are case forms of the same word. Russian simply extends that logic across six cases for every pronoun.

The case a pronoun takes depends on its function: subject of the verb (nominative), direct object (accusative), indirect object / recipient (dative), possession or absence (genitive), instrument or companion (instrumental), or topic following a preposition (prepositional). The meaning is the same — only the form changes.

Я вижу его I see him — accusative · Я дал ей книгу I gave her a book — dative · Он говорит обо мне He's talking about me — prepositional

Personal Pronouns — Declension form changes by case, not gender
Case яI тыyou онhe онаshe мыwe выyou (pl) ониthey
Nominative who? (subject) я ты он она мы вы они
Genitive of whom? without whom? меня тебя егонего after prep еёнеё after prep нас вас ихних after prep
Accusative whom? what? (direct object) меня тебя егонего after prep еёнеё after prep нас вас ихних after prep
Dative to whom? for whom? мне тебе емунему after prep ейней after prep нам вам имним after prep
Instrumental by whom? with whom? мноймною (literary) тобойтобою (literary) имним after prep ей / еюней / нею after prep нами вами иминими after prep
Prepositional about whom? (always after a prep) мне тебе нём ней нас вас них
н- prefix: 3rd-person pronouns (он, она, они) add н- when directly preceded by a preposition — e.g., у него, к ней, с ними. · The Prepositional case always follows a preposition, so н- forms are the only forms used there. · Nominative forms are dimmed — that is the base dictionary form, not a case you apply to something.
Possessive Pronouns agrees with the noun described, not the owner
Pronoun Masculine Feminine Neuter Plural
мойmy / mine · (я) мой моя моё мои
твойyour / yours · (ты) твой твоя твоё твои
егоhis / its · (он / оно) его его его его
еёher / hers · (она) её её её её
нашour / ours · (мы) наш наша наше наши
вашyour / yours · (вы) ваш ваша ваше ваши
ихtheir / theirs · (они) их их их их
Invariable — 3rd person (grayed above): его, её, and их never change form regardless of the noun's gender, number, or case. · Declining — 1st & 2nd person: мой, твой, наш, ваш each have 4 gender/number forms (shown above) and also decline by case like adjectives — e.g., с моим другом (with my friend — instrumental).
Agreement rule for possessives

The form of the possessive reflects the noun being described, not the owner. So "my" takes a different form depending on what is "mine" — one per gender/number in the nominative alone:

мой брат my brother (masc.) · моя сестра my sister (fem.) · моё письмо my letter (neut.) · мои друзья my friends (pl.)

Those four forms then also change by case — у моей сестры (of/from my sister — genitive) or с моим другом (with my friend — instrumental). Third-person possessives (его, её, их) skip all of this — they are invariable in every context.

Key
Personal pronouns — decline by case
Possessive pronouns — agree with noun gender & number
Dimmed cells = invariable (его / её / их never change)