Знаешь его брата - is 'его' in genitive and 'брата' in accusative?

Question

There is this simple sentence I don't quite understand, grammatically;

Он думает, ты знаешь его брата.
He thinks you know his brother.

So based on things like,

Ivan's dog = The dog of Ivan
собака Ивана (nominative, genitive)

I understand that его is genitive (he 'owns' the 'him/brother'; in fact the possessive of он is его) and брата would be the object of 'to know', and hence accusative.

I guess I would write it as (but probably not standard?);

Он думает, ты знаешь брата его.

I just want to check these case roles because masc. animate nouns have the same spelling in genitive and accusative cases (minus minority endings).

Also, его can be a possessive pronoun (genitive of он) or the accusative.

So, my question is: Is my assumption correct, that его is in the genitive and брата is in the accusative?

Answer 1

No, it is incorrect.

«Брата» is indeed in the accusative case.

But «его», as a third person possessive pronoun, is ever the same and does not change by case. That's a nice quirk in the Russian pronominal system that it is useful to be aware of: 1-st and 2-nd person possessive pronouns are, grammatically, adjectives, just like the similar words in the Romance languages, but 3-rd person possessive pronouns never decline.

Update: etymologically, this pronoun does, of course, stem from the word «он» (gen. «его»), which, quite like a noun, declines in cases. In older literature, you would often find that 3-rd possessive pronouns are put after the word they modify, which is, of course, a trace of this origin («сапог его весь промок»). But now no-one would say off-hand that «я отыскал телефон жены» and «я отыскал её телефон» are grammatically parallel, just like no-one would ever utter «я отыскал телефон её» unless under haste etc, when the word order is influenced by something “extra-linguistical”.