Here's why:
* Infinitive: The base form of a verb, usually preceded by "to" (e.g., to eat, to sleep, to run).
* In the sentence: The infinitive "to struggle" acts as the object of the verb "struggled". It tells us *what* they struggled to do.
* "To finish project" is a separate infinitive phrase acting as the object of the preposition "to". It tells us *how* they struggled.