Why the right English translation of Hebrews 1:8 is Thy throne O God and not God is your throne?
As
JChesney pointed out, this is a case of the nominative acting as a vocative of address.
There are two major questions that confront us in the structure of verse 8. First, is the complete absence of the vocative case indicator in the opening address, πρὸς δὲ τὸν υἱόν, Ὁ θρόνος σου ὁ θεὸς. Second, is the question of the subject. So, what are the possibilities?
θεὸς is the nominative spelling rather that the vocative θεέ. Yet, it still functions as a vocative. Such use is common in the New Testament. This is what Wallace refers to as a "nominative for a vocative." In his Greek grammar
“Beyond the Basics - an exegetical syntax of the New Testament,” on page 59, Wallace comments on the use of the nominative for the vocative using Hebrews 1:8 as his example.
Wallace says,
“A substantive in the nominative is used in the place of the vocative case. It is used (as is the vocative) in direct address to designate the addressee. There are three syntactical possibilities for θεός here: as a subject (“God is your throne”), predicate nominative (“your throne is God”), and nominative for vocative (as in the translation above). The Subject and Predicate Nominative translations can be lumped together and set off against the nominative for vocative approach. It is our view that the nominative. for vocative view is to be preferred for the following reasons:
(1) It is an overstatement to argue that if a writer wanted to address God he could have used the vocative θεέ, because no where in the NT is this done except in Matthew 27:46. The articular nominative for vocative is the almost universal choice.
(2) This is especially the case in quoting from the LXX (as in Hebrews 1:8; cf. Hebrews 10:7), for the LXX is equally reticent to use the vocative form, most likely since Hebrew lacked such a form.
(3) The accentuation in the Hebrew of Psalms 45:7 suggests that there should be a pause between “throne” and “God” (indicating that tradition took “God” as direct address).
(4) This view takes seriously the μέν … δέ construction in verses 7–8, while the Subject - Predicate Nominative view does not adequately handle these conjunctions. Specifically, if we read v 8 as “your throne is God” the δέ loses its adversative force, for such a statement could also be made of the angels, viz., that God reigns over them.” End Quote.
The nominative vs. the vocative is indeed a powerful argument. In all other instances where God is addressed in the New Testament, other than Matthew 27:46, God is addressed in the nominative case yet, the force of the address is vocative. In Mark 15:34, Mark rehearses this same account of the crucifixion and he uses the nominative case rather that the vocative in Jesus' address to the Father as Matthew did - Ὁ θεός μου ὁ θεός μου, εἰς τί ἐγκατέλιπές με. This example gives force to the use of the nominative functioning as a vocative. Hebrews. 1:8 is simply another example of this type of grammatical structure.
In verse 9, this same grammatical structure is found yet again in the phrase - διὰ τοῦτο ἔχρισεν σε ὁ θεὸς, ὁ θεός σου ἔλαιον –
“Because of this God, your God has anointed you with oil...” Here again is a case of the nominative functioning as the vocative. The only absolute vocative spelling of address appears in verse 10 where the Father addressing the Son as Lord saying, καί, Σὺ κατ’ ἀρχάς, κύριε –
“And you in beginning Lord...”?
The force of the nominative functioning as a vocative then offers very strong support to the translation
“Thy throne O God.” This in turn establishes the throne as the subject and God as the possessor of the throne. The Son is the God who occupies the throne.