A.T. Robertson Commentary Revelation 11:6

"These have the power to shut the heaven, that it rain not during the days of their prophecy: and they have power over the waters to turn them into blood, and to smite the earth with every plague, as often as they shall desire." — Revelation 11:6 (ASV)

To shut the heaven (κλεισα τον ουρανον). First aorist active infinitive of κλειω. As Elijah did by prayer (1 Kings 17:1; Luke 4:25; James 5:17).

That it rain not (ινα μη υετος βρεχη). Sub-final use of ινα μη with the present active subjunctive of βρεχω, old verb to rain (Matthew 5:45), here with υετος as subject.

During the days (τας ημερας). Accusative of extent of time. In Lu 4:25; James 5:17 the period of the drouth in Elijah's time was three and a half years, just the period here.

Of their prophecy (της προφητειας αυτων). Not here the gift of prophecy (1 Corinthians 12:10) or a particular prophecy or collection of prophecies (Revelation 1:3; 22:7f.), but "the execution of the prophetic office" (Swete).

Over the waters (επ των υδατων). "Upon the waters." As Moses had (Exodus 7:20).

Into blood (εις αιμα). As already stated in 8:8 about the third trumpet and now again here.

To smite (παταξα). First aorist active infinitive of πατασσω, used here with εξουσιαν εχουσιν (they have power), as is στρεφειν (to turn).

With every plague (εν παση πληγη). In 1 Kings 4:8, but with reference to the plagues in Egypt.

As often as they shall desire (οσακις εαν θελησωσιν). Indefinite temporal clause with οσακις and modal εαν (= αν) and the first aorist active subjunctive of θελω, "as often as they will."