I am learning German with our lord and savior Duolingo and came across this sentence:
The cheese is good although it is old.
... which I translated as...
Der Käse ist gut, obwohl es alt ist.
... and Duo said that is wrong, and that the correct answer is:
Der Käse ist gut, obwohl er alt ist.
I think "der Käse" should be an "it/es", not a "he/er". Why is "es" wrong and "er" correct here?