Learn Croatian: Ovdje postoji velika opasnost.

Nouns ending in -ost (like opasnost, meaning danger) belong to a special group of feminine nouns that end in a consonant, known as i-declension nouns. You can see it is being treated as feminine in this sentence because the adjective describing it, velika, takes the standard feminine -a ending.

Is opasnost in the nominative or accusative case here?

It is in the nominative case because it is the subject of the sentence (it is the thing that exists). For these feminine consonant-ending nouns, the nominative and accusative singular forms look exactly the same (opasnost). However, if the phrase were in the accusative case, the adjective would change to veliku. Since we see velika, we know for sure this is nominative.

Why is the subject (velika opasnost) at the end of the sentence?

Croatian has flexible word order and often places the most important piece of new information at the very end of the sentence. While Velika opasnost postoji ovdje is grammatically correct, placing velika opasnost at the end emphasizes the danger itself rather than the location.

Could I just say Ovdje je velika opasnost instead of using postoji?

Yes, using the verb biti (to be) is perfectly fine and translates to 'There is a great danger here.' Using postojati (to exist) makes the sentence sound slightly more formal and emphasizes the actual existence or presence of the threat.