Learn Croatian: Značajna većina ispitanika su odobravali ovaj novi trend. - A significant majority of respondents approved of this new trend.

This sentence demonstrates semantic agreement, which is common with collective nouns like većina (majority). Because većina is followed by the genitive plural ispitanika (respondents), the verb logically agrees with the plural masculine group rather than the grammatical gender of the word većina. You could also use strict grammatical agreement here and say je odobravala, which is feminine singular.

Why is the noun ispitanika in the genitive case?

In Croatian, nouns expressing a quantity, group, or multitude, such as većina (majority), mnoštvo (multitude), or hrpa (bunch), act like quantity words. The noun that tells you what makes up that group must always be in the genitive plural.

How do we know ovaj novi trend is in the accusative case, since it looks exactly like the nominative?

It is the direct object of the verb odobravati (to approve of), which requires the accusative. For masculine inanimate nouns like trend, the accusative case is identical to the nominative. The pronoun ovaj and adjective novi also keep their nominative-like inanimate forms here.

Could I use the perfective verb odobrili instead of the imperfective odobravali here?