quoting dhok, Alkali Metal, Norman, United States:

ἀλεθινὼς ἱστορικὸν βιβλίον περὶ τῆς γλώσσας τῆς ἑλληνικῆς, ᾧ ἥδε γλῶσσα ἔχει μανθάνεσθαι, ζήτεόν εστι. οἷον βιβλίον περὶ τῆς σανσκρίτης ἐστί, καὶ φασιν ὅτι ἀγαθὸν ᾖ, ἀλλὰ δὴ εὔωνον οὔκ ἐστι..
A book about Greek from a really historical perspective that you could learn the language with would be much-desired. Such a book exists for Sanskrit, and I've heard it's good, but it's not cheap.

ἐγώγ' ἐλπίζω τὸν Τϝάβα σὺν ἡμῖν τῇ ἑλληνικῇ τῇ παλαίᾳ ἄρξοντα γράφειν.
I, at least, hope that Twabs will start writing with us in Ancient Greek.

ἀληθινῶς μὲν λέγεις, καὶ τοῦτʼ οὖν σπάνιον· γνησίως δέ βέλτιον εἴη ἄν εἰ οὐ μάλα· γλώσσης δέ
You mean ἀληθινῶς, and that's pretty rare. γνησίως might be better, if not simply μάλα. Also, γλώσσης.

οὐ πιστεύω ὅ τι ἥδε ἐχρῶ καὶ οὐ ἁπλῷ ἡ· ἡ δὲ ἴσως ἂν λέξειας; ἆρα δυνατός ἐστιν ἐν ἀναφορικῷ κόμματι οὕτως χρῆν δέ
I'm not sure why you're using ἥδε and not simply ἡ—perhaps you meant ἡ δὲ? (Can you use δέ like that in a relative clause?)

ζήτεον μὲν ἂν εἴη ζητήτεον καίπερ λέγων πολυάρατον· οὐκ δʼ ἦπου ἂν προκέοιτο εὔωνον
ζήτεον should be ζητήτεον, though I'd say πολυάρατον. οὐκ should probably go before εὔωνον.

ἄρξειν γράφοντα ἡ ἀριστὴ τάξις ἐνθάδε: δοκέω δέ σε ταραχθῆναί γε.
ἄρξειν γράφοντα is the best construction here, and I think you just got confused.