Questions & Answers about On ne vidi opasnost.
Why doesn't the word opasnost have an accusative ending here?
The verb vidi requires a direct object in the accusative case. However, opasnost is a feminine noun ending in a consonant (an i-declension noun). For these specific nouns, the accusative singular form is exactly the same as the nominative singular form. That is why the word doesn't change, unlike standard feminine nouns that would take an -u ending.
How do I know opasnost is a feminine noun when it ends in a consonant?
A massive clue is the -ost ending. This suffix is used to create abstract nouns from adjectives (in this case, from opasan, which means "dangerous"). Nouns ending in -ost are almost universally feminine consonant-ending nouns.
Why is the negation written as two words (ne vidi) instead of one?
Is the pronoun on strictly necessary in this sentence?
No, you could just say Ne vidi opasnost. Croatian verbs have different endings for different subjects, so the "he/she/it" is already built into the 3rd-person singular form vidi. You only include on if you want to emphasize that he (and not someone else) doesn't see the danger, or to clarify who you are talking about if it isn't obvious from the context.