Questions & Answers about Heidän tyttärensä on nuori.
Why do we need heidän here? Can't we just say tyttärensä to mean "their daughter"?
For third-person possessives ("his", "her", "their"), you generally must use the pronoun (hänen or heidän) along with the possessive suffix (-nsä). While you can often drop the pronoun for "my" or "your" (saying just tyttäreni instead of minun tyttäreni), you must keep heidän here so people know whose daughter it is.
Does the suffix -nsä change if the sentence is about "his" or "her" daughter instead of "their"?
No, it stays exactly the same! The third-person possessive suffix is always -nsa or -nsä whether it belongs to one person (his/her) or many (their). Because the suffix itself doesn't change for plural owners, the pronoun in front (hänen or heidän) is doing all the work to tell you exactly who the daughter belongs to.
How does the word tytär (daughter) turn into tyttärensä?
When adding a possessive suffix to a word that ends in a consonant (like the -r in tytär), you attach the suffix to the word's vowel stem. This is the same base you use to build the "genitive" or "of" form (tyttären). So, you take the stem tyttäre- and add -nsä to get tyttärensä.
Why does this word use the -nsä ending instead of -nsa?