Answer:
To please - used as an adverb
Explanation:
The infinitive is the basic form of the verb, and it's always written as to + verb. In the given sentence, the infinitive is to please.
Used in a sentence, the infinitive can act as a noun, adjective, or adverb and play the role of a part of speech other than itself. Here it is used as an adverb.
Adverbs are words and expressions used to modify verbs, adjectives, determiners, clauses, prepositions, or sentences. Adverbs can express many things: manner, place, time, frequency, degree, level of certainty, etc.
The infinitive to please is used as the adverb that modifies the adjective hard, which describes the subject That type of person (What is that type of person like? Hard to please.)
