Learn Dutch: Ken jij de journalist die dit verhaal opschrijft? - Do you know the journalist who writes down this story?

The word die refers back to de journalist. In Dutch, when you want to use "who" or "that" to give more information about a de-word, you use die. If you were describing a het-word, you would use dat instead.

Why is opschrijft written as one word at the end? Shouldn't the verb split into schrijft ... op?

Opschrijven (to write down) is indeed a separable verb. However, the section starting with die is a relative clause, and these clauses push the verb to the very end of the sentence. When a separable verb is forced to the end like this, its two parts glue back together into a single word: opschrijft.

Why does the sentence use ken for "know" instead of weet?

You use kennen when you are talking about being familiar with a person, place, or thing—in this case, knowing the journalist. You only use weten when you are talking about knowing a fact or having information (like knowing his name or knowing where he lives).

Why doesn't the verb have a -t at the end in Ken jij?