Quick answer
In the accusative, animate nouns (people, animals) take the genitive form. Inanimate nouns stay unchanged: Я ви́жу стол, but Я ви́жу бра́та.
The rule in a table
| Noun | Accusative | Why |
|---|---|---|
| стол (thing) | стол | unchanged |
| брат (person) | бра́та | like the genitive |
| соба́ка (animal, f.) | соба́ку | feminine -а → -у anyway |
| друзья́ (plural, people) | друзе́й | like the genitive plural |
| столы́ (plural, things) | столы́ | unchanged |
So the rule bites in two places: masculine singular, and every plural.
Where it feels odd
Dolls, dead people and chess pieces count as animate; a corpse (труп) does not. Grammatical animacy is not biology — it is a habit the language kept.
Я ви́жу бра́та.I see my brother.
Я ви́жу стол.I see a table.
Я зна́ю э́тих люде́й.I know these people.
Я чита́ю э́ти кни́ги.I am reading these books.
Typical mistakes
Я ви́жу брат.
Я ви́жу бра́та.
A person in the accusative takes the genitive form.
Я ви́жу столо́в.
Я ви́жу столы́.
Things stay in the plain plural.
Check yourself
Я жду ___. — I am waiting for my friend.
A person as an object takes the genitive form: дру́га.