Learn Italian: Abbiamo intervistato gli studenti con un lungo questionario. - We interviewed the students with a long questionnaire.

In Italian, most descriptive adjectives go after the noun for an objective, literal meaning (un questionario lungo simply states the fact of its length). Placing it before the noun (un lungo questionario) is a stylistic choice that adds emphasis or a subjective feeling—in this case, perhaps hinting that the survey felt tedious or impressively extensive to those taking it.

If I wanted to use the noun "intervista" (interview) instead of the verb, how would the sentence change?

You would use the expression fare un'intervista a. The sentence would become Abbiamo fatto un'intervista agli studenti. Note that while the verb intervistare takes a direct object (gli studenti), the noun phrase requires the preposition a, which combines with the article to form agli.

How would we spell the plural if they used multiple "long questionnaires"?

It would become lunghi questionari. Remember that adjectives ending in -go add an h in the plural to keep the hard "g" sound. Additionally, because the i in the -io ending of questionario is unstressed, the plural drops the -o and leaves just a single -i, rather than a double -ii.

If we changed the sentence to say "30% of the students were interviewed", how would the verb agreement work?