What part of speech is “difficult”

Question

It is exceedingly difficult to ascertain precisely what is meant by the word 'culture'.

That is a sentence from exercise for English learners where they are asked to determine the part of speech of the word in bold. My decision was that difficult is an adverb in this context because of an adverbial meaning but the author writes that this is an adjective. Who is in the right?

Answer 1

The author is correct in saying it as an adjective. The word in this sentence is a predicate adjective. Is is a linking verb, so the word will either be a predicate nominative or a predicate adjective. In this case, it is describing the pronoun it so it is an adjective. It is difficult.