Ἀθήνᾱζε Α, "Τὰ γὰρ πρόβατα ἐν τῷ αὐλίῳ ἑστίν"

[…] ο μεν γαρ Φιλιππος τα προβατα θεραπευει· τὰ γὰρ πρόβατα ἐν τῷ αὐλίῳ ἑστίν.

This sentence appears at page 55 in the Italian edition of Ἀθήνᾱζε Α. Τὰ προβατα seems nominative plural of the neuter noun το προβατον, this is quite clear form the context. Is the use third singular of εἰμί (instead of εἰσίv) a typo?

No, it’s right. See Herbert Weir Smyth, A Greek Grammar for Colleges

[*] 958. A neuter plural subject is regarded as a collective (996), and has its verb in the singular: ““καλὰ ἦν τὰ σφάγια” the sacrifices were propitious” X. A. 4.3.19.
N.—The neuter plural seems to have been originally in part identical in form with the feminine singular in α_, and to have had a collective meaning.

Thank you, @bedwere. Unfortunately this is not signaled in the marginal notes. There’ll probably be an explanation later in the book.

An oversight, probably. I have taught Latin out of one book, however (Jenney) which does it deliberately, in the readings introducing a construction or vocabulary which then shows up a chapter or two later, no doubt expecting the teacher to fill in the gap, but in the hope that students might be able to figure it out from context, which sometimes they do. I always called it a “sneak preview of coming attractions.”