Final answer:
The infinitive phrase 'To be persistent' acts as a predicate nominative, renaming the subject 'persistent'.
Explanation:
The infinitive phrase 'To be persistent' in the sentence 'To be persistent is a sign of stubbornness' plays the role of a predicate nominative.
It renames the subject 'persistent' and functions as an equal sign after the linking verb 'is', indicating that being persistent is the same as being a sign of stubbornness.
Understanding different roles of infinitive phrases like this enhances sentence analysis and comprehension.
Learn more about Infinitive phrase role in sentence structure here:
https://brainly.com/question/15315457