An infinitive is a verbal noun (e.g., to eat, to run, to go). It expresses aspect and voice like other (finite) verbs, but does not indicate person or number. It can take an object, but usually does not explicitly express a subject.
οὓς οὐκ ἔξεστιν φαγεῖν εἰ μὴ τοῖς ἱερεῦσιν, (Mrk 2:26)
it is not lawful for anyone to eat except the priests