A substantival possessive pronoun is a personal determiner that is used without an accompanying noun, such as "mine," "his" or "hers," "yours," "ours," and "theirs." In English, substantival possessive pronouns sometimes have a different form than standard personal pronouns with a noun: "my" becomes "mine," as in "My wallet is in my bag." "Oh, where is mine?" In German, however, the substantival variant depends upon the inflective ending of the original pronoun that it is replacing.
Ich nehme mal an, Ihre Schulzeit liegt ähnlich weit zurück wie meine.
I take it your time at school dates, likewise, back as far back as mine.
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Since he is referring to his time at school (feminine noun, singular nominative case: die Schulzeit), the substantival possessive pronoun is inflected accordingly: meine.
Dafür könnt' ich sie bei der Schulbehörde anzeigen, aber dann stünde dein Wort gegen ihres.
I could report her for that to the school authorities, but then it would be your word against hers.
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Although the possessive of das Wort would be ihr Wort, the substantival possessive pronoun adds -es to the neuter singular nominative case. If the substantival possessive pronoun were standing in for a masculine noun, it would add -er for the masculine case. You will most often encounter -es endings on pronouns in the genitive case, so best keep a sharp eye out for this exception!
Visit Yabla German and search for examples of nouns that you can practice turning into substantival possessive pronouns. Check out this site to further practice your pronoun skills!