Learn Dutch: Het is onduidelijk waarom deze horoscoop zo negatief is. - It is unclear why this horoscope is so negative.

The prefix on- is the Dutch equivalent of the English 'un-' or 'in-'. It attaches to the adjective duidelijk (clear) to form onduidelijk (unclear). It is highly productive, meaning you will often see it added to existing adjectives and participles to create their exact negative opposites.

Why is the verb is placed at the very end of the sentence?

The word waarom (why) introduces a subordinate clause in this sentence. In Dutch subordinate clauses, the conjugated verb is pushed to the end of the clause. That is why the sentence ends with waarom deze horoscoop zo negatief is rather than keeping the standard main-clause word order.

Could I use dit instead of deze for 'this horoscope'?

No, because horoscoop is a de-word (de horoscoop). When pointing to a de-word, 'this' is always translated as deze. You only use dit for het-words (like dit huis).

Why doesn't the adjective negatief get an -e ending here?