Marian had plenty of work to finish
In the correction form, it says that the function of to finish is adverbial.
But why it is not an adjective? Which work? The work to finish. So it is an adjective!!
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Sign up to join this communityYou're right in your analysis, but not exactly correct in the terminology.
The phrase is not an adjective, but because it modifies a noun, it is adjectival.
It has no function like an adverb, so it's not an adverbial, and the answer key is wrong.
Marian had plenty of work [to finish].
The bracketed element is neither an adjective nor an adverb, but an infinitival relative clause modifying "plenty of work".
It has a modal meaning comparable to that expressed in finites by "can" or "should", and is thus comparable to Marian had plenty of work that she could / should finish.
Note that 'adjective' and 'adverb' are parts of speech, not clause types.