Quote Originally Posted by Hoax View Post
А что случилось с его чрезвычайной комиссией? - Instrumental
Когда я буду в Москве, я позвоню его сестре. - Dative
В течение этого времени я думал о моём друге Владимире. - Prepositional

1. Possessive must agree in gender, number and case with the noun they modify (the thing possessed). Thus, for example, “my sister” will be "моя сестра" whether the speaker (“I”) is masculine or feminine.
2. Note, however, that the possessives that correspond to the third person personal pronouns (singular and plural) are unchanging in form. Corresponding to both он and оно is его (“his, its”); corresponding to она is её (“her, hers, its”); and corresponding to они is их (“their, theirs”).
Thank you for your response. The bold type in Part 2 of your answer explains the problem well