A.T. Robertson Commentary Revelation 14:3

"and they sing as it were a new song before the throne, and before the four living creatures and the elders: and no man could learn the song save the hundred and forty and four thousand, [even] they that had been purchased out of the earth." — Revelation 14:3 (ASV)

They sing as it were a new song (αιδουσιν ως ωιδην καινην). See 5:9 for this phrase (cognate accusative) save that here ως (as if) is added. There the new song was sung by the four living creatures and the elders, but here "before" (ενωπιον) them and so apparently by the throng who were themselves redeemed by the Lamb.

No man could learn the song save (ουδεις εδυνατο μαθειν την ωιδην ε μη). Imperfect (εδυνατο) of δυναμα and second aorist (ingressive) active infinitive of μανθανω. In 5:9-12 the angels join in the song. In 15:3 it is the Song of Moses and the Lamb.

Even they that had been purchased out of the earth (ο ηγορασμενο απο της γης). Perfect passive articular participle of αγοραζω, purchased by the blood of the Lamb (5:9), masculine plural in apposition with χιλιαδες (thousands) feminine plural (7:5,8; 14:1). Απο (from) here, though εκ (out of) in 5:9. The 144,000 are not yet separated from the earth (John 17:15). Whether the 144,000 here are identical with that number in 7:4-8 or not, they must embrace both men and women.