A.T. Robertson Commentary Revelation 11:17

"saying, We give thee thanks, O Lord God, the Almighty, who art and who wast; because thou hast taken thy great power, and didst reign." — Revelation 11:17 (ASV)

O Lord God (Κυριε ο θεος). Vocative form κυριε and nominative form ο θεος (vocative in use). See 1:8; 4:8 for this combination with ο παντοκρατωρ (the Almighty). For ο ων κα ο ην (which art and which wast) see 1:4,8; 4:8; 16:5.

Thou hast taken (ειληφες). Perfect active indicative of λαμβανω, emphasizing the permanence of God's rule, "Thou hast assumed thy power."

Didst reign (εβασιλευσας). Ingressive first aorist active indicative of βασιλευω, "Didst begin to reign." See this combination of tenses (perfect and aorist) without confusion in 3:3; 5:7; 8:5.