at the first of the Mounce BBG 3rd edition pages(before the table of contents), He gives a quote from LXX
ΨΑΛΜΟΙ ΙΗ 8-10, 15
the interesting part for me is that, at the first-half of the second line (it appears to be verse 9)
It reads, τἁ δικαιώματα κυρίου εὐθεῖα, εὐφραίνοντα καρδἰαν·
it seems to me that εὐθεῖα is a female singular adjective and is predicating the neuter plural noun τἁ δικαιώματα. is it possible for a female adj to predicate a neuter or masculine noun? or is it just the correct colloquial Greek at the time, or I just miss something here?
interestingly, Mounce seems follow the Rahlf’s edition(German Bible society), and the one I have (Zoe brotherhood/Greek Orthodox) use the neutral form εὐθέα.
Unless I am also missing something, εὐθεῖα appears to be some sort of error. Unless Rahlf is for some reason following an accented (and therefore late?) manuscript, it would appear to be a modern typo. Both Brenton and Α.Δ.Ε.Ε. have εὐθέα, as do various on-line editions.
If it is a mistake, Mounce should be told.
brenton sure a big name and a classic.. but what does A.Δ.Ε.Ε stand for? sry for my ignorance 
Ἔκδοσις τῆς Ἀποστολικῆς Διακονίας τῆς Ἐκκλησίας τῆς Ἑλλάδος.
oh I see.. so thats how we call it..
The Rahlfs version seems to have a great deal of manuscript support:
https://books.google.com/books?id=4KA0AQAAMAAJ&pg=PA272
EDIT: Here it is in the Codex Sinaiticus:
well surely it is hard to discuss the topic, without bringing textual criticism I think, my mistakes then
is it possible that εὐθεῖα is a different word but related to εὐθύς?
or instead we could just accept Greek as Greek 
See LSJ under εὐθής:
εὐθής, ές, only in later LXX translators for εὐθύς A, LXX 1 Ki.29.6, Ps. 118(119).137; βιβλίον τοῦ εὐθοῦς the Book of the Righteous (Jashar), ib.2 Ki.1.18: neut. pl. εὐθεῖα ib. 2 Es.19.13, cf. interpol. in Thom. Mag. p.165 R., Suid. s.v. εὐθυγενής.
This surprised me, because I had already looked in Rahlfs for other instances of εὐθεῖα and didn’t see any for the neuter pl.
Contra the LSJ, for 2 Es. 19.13 Rahlfs uses εὐθέα, not εὐθεῖα. However look at that verse in the Sinaiticus:
Aside: The original copyist did not use the ι in οροϲ ϲιναι and it had to be supplied by a latter hand. I believe that the modern Greek for ϲιναι is Σινά.
The use of the feminine for the neuter is a pure Hebraism, which occurs principally in the Psalms.
F. C. Conybeare, George Stock p. 53
None of Conybeare’s examples involve a change from plural to singular.
to think of it, Hebrew does not have a neuter type noun, does it? so they would switch it to feminine corresponding to their native-mindset. even though it would seems foreign for the Greeks?
A singular for a neuter plural is a Greek thing, nothing to with the LXX or Hebrew-Greek translations.
Look at his examples though (https://books.google.com/books?id=WCM2AAAAMAAJ&pg=PA53):
They don’t look at all similar to this. None of them involve a broken concord. εὐθής seems far more likely to me, especially as it appears elsewhere in the manuscripts.
And I think (I’m not sure) that this is the Thom. Mag. reference in the LSJ: https://books.google.com/books?id=Mm3RAAAAMAAJ&dq=Thomas%20Magister&pg=PA383#v=onepage&q&f=false
Joel,
I looked at the examples. I agree they don’t look like τὰ δικαιώματα κυρίου εὐθεῖα Psalm 18:9. That doesn’t solve anything. Albert Pietersma who translated the LXX Psalter for NETS might be willing to help out but I haven’t seen any recent posts from him at the other place.
What were looking at here is noise level in terms of LXX grammatical peculiarities. You should read Emanuel Tov’s several books and numerous articles on LXX and Hebrew Text. Good place to start is:
The Text-Critical Use of the Septuagint in Biblical Research (Third Edition, Completely Revised and Enlarged; Winona Lake, IN: Eisenbrauns, 2015.