I have a simple question concerning the following sentences:
Die Katze trinkt ihr Wasser.
and
Der Hund trinkt sein Wasser.
I would like to get a better understanding as to why "ihr" and "sein" are used the way the are. Are they interchangeable or is there a particular reason they are used in this way?
In German, the choice of the possessive article in the third person (sein, ihr) depends on the gender of the noun it is referring to. In your examples, the possessive articles refer back to the subject and the gender of the noun is clearly indicated by the forms of the definite article (der, die).
Die Katze trinkt ihr Wasser. (feminine)
Der Hund trinkt sein Wasser. (masculine)
Note that sein can also refer to neuter nouns.
Das Huhn trinkt sein Wasser. (neuter)
However, the ending of the possessive article depends on the noun that follows it.
Die Katze mag ihren Kratzbaum und der Hund mag seinen Knochen. (accusative masculine)
Die Katze sucht ihre Beute und der Hund sucht seine Beute. (feminine)
