Breakdown of Gehört der Schlüssel Peter? Ja, das ist seiner.
sein
to be
ja
yes
der Schlüssel
the key
Peter
Peter
Questions & Answers about Gehört der Schlüssel Peter? Ja, das ist seiner.
Why does the sentence use seiner instead of just sein?
Here, the word for 'his' stands completely on its own, replacing the noun phrase sein Schlüssel. When possessive words like mein or sein replace a noun rather than sitting next to one, they need a strong ending to show the gender of the missing noun. Because der Schlüssel is masculine and is the subject of the second sentence (nominative case), sein gets the masculine -er ending, becoming seiner.
If the key belonged to Maria instead of Peter, would we still use seiner?
No, the root word would change to match the new owner. For a female owner ('her'), the base word is ihr. We then add that same masculine -er ending because the object being owned (der Schlüssel) is still masculine. The answer would be: Ja, das ist ihrer.
Why does the answer start with das ist? Since Schlüssel is masculine, shouldn't we use er?
When you are identifying or pointing to something with phrases like 'that is...' or 'this is...', German uses the fixed phrase das ist regardless of the gender of the object. You only need to worry about the masculine gender when choosing the ending for seiner.
What case is Peter in the first sentence?