Question

to question is an effective strategy when reading a difficult text. subject predicate nominative adjective

Explanation:

In the sentence "To question is an effective strategy when reading a difficult text", the phrase "To question" functions as the subject. A subject is what the sentence is about, and here "To question" (the infinitive phrase) is the entity that is being described as an effective strategy. The "predicate nominative" would be a noun or pronoun that renames the subject, but here "an effective strategy" is the predicate nominative, not "To question". An adjective describes a noun, and "To question" is not an adjective. So "To question" is the subject.

Answer:

subject