Lexical Definition

‎ναί‎ naí (nahee) yes G:PRT
a primary particle of strong affirmation

even so, surely, truth, verily, yea, yes.

Usage: 3 forms in 39 verses
Form Translated As     In Verse(s)
ναὶ Yes Matt.5.37Matt.9.28Matt.11.9Matt.11.26Matt.15.27Mark.7.28Luke.7.26Luke.10.21Luke.11.51Luke.12.5John.11.27John.21.15John.21.16Acts.5.8Rom.3.292Cor.1.172Cor.1.172Cor.1.182Cor.1.192Cor.1.19Phil.4.3Jas.5.12Jas.5.12Rev.1.7Rev.16.7Rev.22.20
ναι Yes LXX.Gen.17.19LXX.Gen.42.21LXX.Job.19.4LXX.Isa.48.7
ναί Yes Matt.5.37Matt.13.51Matt.17.25Matt.21.16Acts.22.272Cor.1.20Phlm.1.20Rev.14.13Rev.22.20

Abbott-Smith

ναί, particle of affirmation,
yea, verily, even so; in answer to a question: Mt 9:28 13:51 17:25 21:16, Jo 11:27 21:15, 16, Ac 5:8 22:27, Ro 3:29; seq. λέγω ὑμῖν, Mt 11:9, Lk 7:26; repeated for emphasis, ναὶ ναί (opp. to οὒ οὔ): Mt 5:37; ἤτω ὑμῶν τὸ ναὶ ναί, Ja 5:12; ν. καὶ οὔ, II Co 1:18, 19; ἵνα ᾖ . . . τὸ ναὶ ναί, II Co 1:17; τὸ ν., II Co 1:20; in assent to an assertion: Mt 15:27, Mk 7:28, Re 14:13 16:7; in confirmation of a previous assertion: Mt 11:26, Lk 10:21 11:51 12:5, Phl 4:3, Phm 20; in solemn asseveration: Re 1:7 22:20.†

Abbott-Smith References

Book Refs
Matthew 5:37 · 9:28 · 11:9 · 11:26 · 13:51 · 15:27 · 17:25 · 21:16
Mark 7:28
Luke 7:26 · 10:21 · 11:51 · 12:5
John 11:27 · 21:15 · 21:16
Acts 5:8 · 22:27
Romans 3:29
2 Corinthians 1:17 · 1:18 · 1:19 · 1:20
Philippians 4:3
Philemon 1:20
James 5:12
Revelation 1:7 · 14:13 · 16:7 · 22:20

Liddell-Scott-Jones

ναί,

adverb, used to express strong affirmation, yea, verily, in [Refs 8th c.BC+] yea thee, [ναί 4th-5th c.AD: Choeroboscus Grammaticus “Scholia in Theodosii Canones” 2.85. ">Refs 5th c.BC+]

__2 frequently in oaths, ν. μά yea by.., see at {μ; ν}. alone, ν. τὰν Κόραν [Refs 5th c.BC+]; ν. πρὸς θεῶν ἀρήξατ᾽ [Refs 5th c.BC+]
__II in answers, yes, τοῦτ᾽ ἐτήτυμο; Answ. ν. [NT+5th c.BC+]
__II.2 ν. followed by ἀλλά, etc., to mark a qualified assent, yes, but.., [Refs 4th c.BC+]; ν., κατὰ σχολήν γε ἴσως· οὐ μήν.. [Refs 5th c.BC+]

LSJ Scripture Refs: Matt.5.37