"Which cat is yours?"
Translation:Welche Katze ist deine?
The German possessive pronoun deiner is declined in order to match case, gender and number of the associated object (i.e. the noun and its associated possessive determiner that is replaced.). Deiner is the base form that is to be found in dictionaries. All base forms are singular, nominative, masculine. Deine (etc.) is just a declined form of deiner.
Back to the example sentence:
- Instead of saying "Welche Katze ist deine Katze?", deine Katze needs to be replaced by the appropriate variant of deiner in order to avoid repetition. Deine Katze [die Katze!] is feminine, singular, nominative. Thus the appropriate form of deiner is deine.
Here's the declension pattern that you have to memorize [all forms are quite similar to the possessive determiner dein. Don't mix that up]:
|Sg. Maskulinum||Sg. Femininum||Sg. Neutrum||Plural|