"denen" vs. "dessen"
I'm having trouble understanding when I need to use the word "denen" and when to use "dessen".

For example, "Wollt ihr noch eins von denen?" and "Ich bin mir dessen nicht sicher".

Why is it "denen" in the first example sentence and "dessen" in the second one? How do I know when to use which?

Question

"denen" vs. "dessen"
I'm having trouble understanding when I need to use the word "denen" and when to use "dessen".

For example, "Wollt ihr noch eins von denen?" and "Ich bin mir dessen nicht sicher".

Why is it "denen" in the first example sentence and "dessen" in the second one? How do I know when to use which?

Answer

"Dessen" is genitive (singular), "deren" dative (plural). ("Von" always requires dative, the construction "sich ... sicher sein" genitive.)