"Elle écrit pendant sa jeunesse."
Traduction :Sie schreibt während ihrer Jugend.
Because "ihr" is a possessive feminin article and "sein" is a possessive masculine pronoum. You can memorize it with english :
- ihr = her (both have an R)
- sein = his (both have an S)
So when the personal pronoun is "Sie", you have to use "ihr" : for example, "Sie trägt ihr Kleid" : she's wearing her dress.
And when the personal pronoun is "Er", you have to use "sein" : for example, "Er geht mit seinem Hund spazieren" : He's going out with his dog.
Make sure to do the right accord at the end of "ihr" or "sein" according to the genre of the word behind and the case (nominativ, accusativ, dativ, genitiv) ! I hope it's useful, good luck :)