I can not get "dessen" and "deren" right.
Can anyone help? Is there a simple trick I am not getting? Oh, you can add "wessen" to that list.
Let me give it a try: The English relative pronoun "whose" can be either translated as "dessen" or "deren". You need to figure out what the relative pronoun is referring to. If it refers to a masculine or neuter noun, you translate it with "dessen". If it refers to a feminine or a plural noun, you translate it with "deren". "Wessen" is a question word used when you ask who something belongs to. "Wessen Hund ist das?"
"dessen" refers back to a singular masculine/neuter noun: der Mann, dessen Frau ich kenne. "deren" is used for singular femine nouns or plural nouns: die Frau, deren Mann ich kenne; die Familien, deren Kinder auf diese Schule gehen. This pronoun is also used in genitive in constructions like: Beweise, anhand derer der Täter überführt werden konnte. Etwas, dessen ich mich nicht erinnern kann.
"wessen" is corresponding pronoun used for questions: Wessen Buch ist das? Wessen Hose ist das?
I found this site really useful for getting to grips with all those tricky German grammar rules : http://www.lsa.umich.edu/german/hmr/grammatik/Adjektive/potatoes/adjdiag.html
The link takes you to a diagnostic test, but there are lots of ways of using the site which can help with specific issues.
there is a table here: https://en.wiktionary.org/wiki/dessen As you can see, deren and dessen are genitive forms. Deren is either plural or feminine. Dessen is singular, either masculine or neutral.
Note how this is similar to nominative and accusative, where feminine and plural forms are both "die", while the masculine and neuter forms differ from that.