Possessive pronouns     German English     feminine masculine neutral     Nominative ihre ihrer ihres theirs   Genitive not used * not used * not used * of theirs   Dative ihrer ihrem ihrem to theirs   Accusative ihre ihren ihres theirs               Nominative ihre ihre ihre theirs   Genitive not used not used not used of theirs   Dative ihren ihren ihren to theirs   Accusative ihre ihre ihre theirs

* In common language, the Genitive of the substantival possessive pronoun is not used. There is another form of a possessive that could be used (der ihrige, die ihrige, das ihrige), but it sounds really antiquated. If you want to form a Genitive of ihre it is mostly done like in English.

Das ist der Schlüssel von ihrem (Auto etc.). This is the key of theirs. (their car etc.)

The same could be said with a real Genitive and ihrige (it's just not used commonly):

Das ist der Schlüssel des ihrigen. This is the key of theirs.

Example: die Schachtel (= box) possessed object => feminine, singular

Ich sehe meine, aber ihre nicht. I see mine but not theirs.

Example: das Kleid (= dress) possessed object => neutral, singular

Du kannst mit ihrem hingehen. You can go with theirs.

Example: die Häuser (= house) possessed object => neutral, plural

Ich sehe meine, aber ihre nicht. I see mine but not theirs.