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?