Breakdown of Hier ist das Kabel, von dem du mir erzählt hast.
sein
to be
du
you (informal singular)
hier
here
haben
to have
Questions & Answers about Hier ist das Kabel, von dem du mir erzählt hast.
Why do we use von dem here instead of just das?
In English, you can drop the relative pronoun (the cable you told me about) or put the preposition at the end. German does not allow this. The preposition von belongs with the verb erzählen (to tell about), and in a relative clause, the preposition and its relative pronoun must stay together at the very beginning of the clause as von dem.
Why is the relative pronoun dem? Isn't das Kabel neuter?
Yes, das Kabel is neuter, so the relative pronoun starts out as das. However, the case of the relative pronoun depends on its grammatical role inside the relative clause. Because it immediately follows the preposition von, which always requires the dative case, the neuter das changes to the dative dem.
Could I use wovon here instead of von dem?
No. You use words like wovon when referring to an abstract idea, an entire previous sentence, or an indefinite word like etwas or alles. Because you are referring to a specific, physical noun (das Kabel), you must use the preposition plus the standard relative pronoun (von dem).
Why does the helper verb hast go at the very end, even after erzählt?