One topic that tends to intimidate students learning German is the use of these four words: Wessen, dessen, deren, and denen. They are indeed tricky, but they make it possible to construct elegant sentences and are therefore very good to get acquainted with!
Wessen, dessen, and deren can all be translated as "whose." However, unlike "whose" they are each only correct in certain situations. Wessen is related to wer, wen, and wem, i.e. it is asking "who?," but it is possessive. It is used when the "who" of the sentence is not known and could therefore be any gender:
Sach ma [Sag mal], wessen Freundin bist du eigentlich?
Tell me, whose friend are you actually?
Caption 45, Die Pfefferkörner - GerüchteküchePlay Caption
Es dachte auch nicht darüber nach, wessen Haferbrei das war.
She also didn't think about whose oat porridge it was.Play Caption
Dessen is used when the person or noun in question has already been named or referred to, and is masculine or neuter:
Acht Minuten später schickt der Paulianer geschickt Julian Schieber,
Eight minutes later the "Paulianer" skillfully sends Julian Schieber
dessen Schuss aber knapp vorbeigeht.
whose shot, however, just misses.
Captions 10-11, Fußball - U21-NationalmannschaftPlay Caption
Deren is used when the person or noun in question has already been named or referred to, and is feminine or plural:
Seine königliche Hoheit, der Prinz,
His Royal Highness the Prince
wird die Frau heiraten, deren Fuß in diesen Glasschuh passt.“
will marry the woman whose foot fits in this glass slipper."
Captions 25-26, Märchenstunde - Das AschenputtelPlay Caption
Wessen, dessen, and deren are all genitive relative pronouns. The relative pronoun denen, however, is used for relative clauses involving a plural noun in the dative case, with prepositions such as mit, auf, nach, or bei.
Es macht einfach Spaß, mit denen Zeit zu verbringen.
It is simply fun to spend time with them.
Caption 36, Curly Horses - Pferdeglück auch für AllergikerPlay Caption
Und hier sehen wir zwei alte Kutschen,
And here we see two old coaches,
auf denen man im Sommer Kutschfahrten machen kann mit der Familie zusammen.
upon which you can take coach rides together with you family in the summer.
Captions 37-38, Berlin - Domäne DahlemPlay Caption
Searching for any of these words on Yabla German will bring up a lot of examples that can help you solidify your understanding of the topic. You can additionally watch our videos featuring German teacher Barbara and her students working on relative clauses, starting with this one.