(see Wackernagel, Vorlesungen über Syntax, vol. I, pp. 257ff.).13
To be is εἶναι in Attic, ἦναι in Arcadian, ἔμμεναι in Lesbian, ἦμεν in Doric. To be is esse in Latin, ezum in Oscan, erom in Umbrian. In both <the Greek and Latinate> languages the modi finiti were already fixed and were common property, while the ἔγκλισις ἀπαρεμφατικός still retained its varying peculiarities of dialect. We consider this state of affairs an indication that the infinitive has a preeminent significance in language as a whole. The question remains whether this persistence of the infinitive forms stems from the fact that the infinitive represents an abstract and late verbal form, or whether it names something that lies at the foundation of all inflections of the verb. On the other hand, it is right to warn us to be on our guard against the infinitive word form, for precisely this form, seen grammatically, communicates the least of the verb’s meaning.
But we are still far from having fully clarified the word form that we are discussing, at least if we pay attention to the form in which we ordinarily go about saying “to be.” We say das Sein. Such a manner of speaking results when we transform the abstract infinitive form into a substantive by placing the article in front of it: τὸ εἶναι. The article is originally a demonstrative pronoun. It means that what is indicated stands and is for itself, as it were. This naming that demonstrates and indicates always has a preeminent function in language. If we just say sein, then what we have named is already indefinite enough.
13. Jacob Wackernagel, Vorlesungen über Syntax, mit besonderer Berücksichtigung von Griechisch, Lateinisch und Deutsch, vol. 1 (Basel: Emil Birkhäuser, 1920). Wackernagel discusses the infinitive in general on 257–65. See 257 for his explanation of παρεμφαίνω, 257–8 for the various forms of the infinitive of “be” in Greek and Latinate languages, and 258 for the phrase that Heidegger quotes.