Demonstratives do not agree in Case?

Latin Learning Latin
1

My text says that demonstratives (such as IS) do not agree
in ‘Case’ when acting as pronouns.

‘Case’ means modes of the declension such as Accusative or Dative

If the original noun the demonstrative is replacing is in the Accusative
then IS which is replacing it must be in the Accusative. You could not
use IS in the Dative if the original noun was in the Accusative.

Then why does the rule in the book say that they do not agree in case?


What am i missing?

Thanks.

2

when used as a demonstrative it will agree just like an adjective:

Is vir est amicus meus. This man is my friend.

when used as a pronoun, which it usually is, case is determined by its function just like a noun:

Is est amicus meus. He is my friend.
Eum amo. I love him.

3

Could it be that IS, EA and ID do not agree in case when
they are pesonal/possesive pronouns? Could that be what
the book is refering to?

Thanks.