631. Ἕως (while, so long as, until), likewise Ἔστε, Mέχρι οὗ, Mέχρι, Ἄχρι [οὗ], (until, so long as), introduce relative clauses of time. The indicative states a fact, present or past; the subjunctive (with ἄν, sometimes without) and the optative (without ἄν) have the same force as in ὅς clauses (§ 615.a & b).
In the sense of while, so long as, they naturally take one of the tenses of continuance (present, imperfect, perfect); in the sense of until they generally take the aorist
Ἕως μένομεν αὐτοῦ, σκεπτέον μοι δοκεῖ ὅπως, κτλ.
While we remain here, it seems to me we must consider how, etc.
Xen. Anabasis 1.3.11
ἕως μὲν ἐτῑμᾶτο, πιστὸν ἑαυτὸν παρεῖχεν.
So long as he was honored, he showed himself faithful.
Προσμείναντες ἕως τοὺς νεκροὺς ἀνείλοντο οἱ προσήκοντες
waiting until their kinsmen had taken up the dead
Xen. Hellenica 2.4.7
ταῦτα ἐποίουν μέχρι σκότος ἐγένετο.
This they kept doing till darkness came on.
Xen. Anabasis 4.2.4
Ἕως δʼ ἂν οὖν ἐκμάθῃς, ἔχʼ ἐλπίδα.
But keep hope at any rate until you learn the whole.
Soph. Oedipus the King 834–835
ἕως ἂν σῷζηται τὸ σκάφος, τότε χρὴ καὶ ναύτην καὶ κυβερνήτην προθῡ́μους εἶναι.
While the boat is safe, then ought both sailor and pilot to be zealous.
μέχρι ἂν ἥκω, αἱ σπονδαὶ μενόντων.
Let the truce continue till l return.
Xen. Anabasis 2.3.24
ἐπίσχες ἔστʼ ἂν καὶ τὰ λοιπὰ προσμάθῃς.
Wait till thou hast also learned the rest.
Aesch. Prometheus Bound 697
Ἔδοξεν οὖν προϊέναι ἕως Κῡ́ρῳ συμμείξειαν.
They decided to go on, until they should join Cyrus.
Xen. Anabasis 2.1.2
περιέμενε μέχρι ἔλθοι.
He waited for him to come (till he should come).
Xen. Hellenica 1.3.11
a. A ἕως clause has the indicative also when the action is known to be impossible, because dependent on a clause that implies unreality.
Ἐπισχὼν ἂν, ἕως οἱ πλεῖστοι τῶν εἰωθότων γνώμην ἀπεφήναντο, ἡσχυχίᾱν ἂν ἦγον.
I should have waited, until most of the several speakers had set forth their view, and should have kept still.
b. Some of the above clauses with ἕως, etc., and the subjunctive or optative imply both purpose and condition, while at the same time they are primarily temporal.