Breakdown of Тёща очень гордится своей дочерью.
очень
very
дочь
daughter
свой
one's own
тёща
mother-in-law (wife's mother)
Questions & Answers about Тёща очень гордится своей дочерью.
Why does the word for daughter change to дочерью here?
Дочь is an irregular feminine noun. In all cases except the Nominative and Accusative singular, it adds the stem extension -ер-. Because the verb гордиться requires the Instrumental case, we add the regular feminine Instrumental ending -ью to this extended stem, making it дочерью.
Why do we need the Instrumental case after гордится?
The verb гордиться (to be proud of) always requires the object you are proud of to be in the Instrumental case. You can think of it as expressing pride by means of someone or something.
Could I use её instead of своей to say her daughter?
You should use своей here. The word свой is a reflexive possessive pronoun meaning one's own and it refers back to the subject of the sentence (тёща). If you used её, it would sound like the mother-in-law is proud of someone else's daughter.
Does тёща mean any mother-in-law?
No, Russian has specific words for in-laws depending on whose side of the family they are on. Тёща specifically means a wife's mother. A husband's mother is called свекровь.