Questions & Answers about Папин друг часто спорит с дядиной женой.
дядя is a masculine word, so why does дядиной have a feminine ending?
Possessive adjectives like дядин behave just like normal adjectives: they agree in gender, number, and case with the noun they describe, not the person who owns it. Here, it describes женой (wife), which is feminine, so the adjective must take a feminine form.
How do we know to use the -ин suffix to make папин and дядин?
The suffix -ин is used for people (and animals) whose names or titles end in -а or -я, even if they are masculine words like папа and дядя. If the base noun ended in a consonant, you would use the -ов suffix instead.
Why does дядиной end in -ой? How does the mixed declension work here?
The phrase is in the instrumental case because of the preposition с. Possessive adjectives have a mixed declension: they look like nouns in the nominative case, but in oblique cases like the feminine instrumental, they use standard adjective endings. That is why дядин gets the regular feminine instrumental adjective ending -ой to match женой.
Why do we need the preposition с? Doesn't the instrumental case alone already mean 'with'?