Learn Polish: Część respondentów zmieniła decyzję, zanim opublikowano dane. - Some of the respondents changed their decision before the data was published.

The verb must agree grammatically with the main subject of the sentence, which is the word część (a part). Since część is a feminine singular noun, the verb takes the feminine singular form zmieniła. The verb completely ignores the genitive plural noun respondentów that follows it.

Why does the Polish sentence use the singular decyzję when the English translation implies multiple people changing their minds?

In Polish, when a group of people each make one choice or possess one item, the singular form is heavily preferred. Using the plural decyzje would imply that each individual respondent changed multiple decisions.

What exactly is the verb form opublikowano?

It is the impersonal past tense form, created by adding -no or -to to the verb stem. It is used when an action happened in the past but the doer is unspecified or irrelevant. It translates smoothly to the English passive voice ('the data was published'), but grammatically it acts more like 'an unknown entity published the data'.

What case is the word dane in this sentence?

It is in the accusative case. Even though opublikowano translates to the passive voice in English, in Polish it acts as an active impersonal verb form, meaning it takes a direct object in the accusative. Because dane (data) is a non-masculine plural noun, its accusative form happens to look exactly like its nominative form.