"We are eating his cheese."

Translation: Wir essen seinen Käse.

[DanielTKM]

Wir essen seinen Kase
but
Wir trinken sein Bier

... how do we decide to use "seinen" or "sein"?

[stepintime]

It's because "der Käse" is masculine, and "das Bier" is neutral.

Thus: accusative = seinen Käse, sein Bier, seine Limonade (lemonade, feminine).

[hosejnali]

Akkusativ/Plural