The aorist tense (in Rev 19.6) is the most peculiar to Greek idiom. It is the indefinite tense (aóristos, unlimited, undefined). It has no essential temporal significance, only in the indicative does it have a time relation, usually indicating a past action, but not always. The aorist views an action as a single whole, but may contemplate it from different angles. The aorist states the fact of an occurring action with little or no emphasis on time or duration. This means, that trying to put the blame on a translator for relying on his interpretation in rendering the tense would be like trying to define the indefinite aorist with a fixed meaning to be applied at all times, an impossibility.
Grammarians Dana and Mantey wrote: "Robertson [notable grammarian] calls attention to the difficulty of obtaining an accurate translation of the aorist. To attempt to translate it invariably by the simple past of the English would, in the majority of cases, do violence to the real shade of meaning intended to be conveyed. We should take into consideration the significance of the tense, find its relation to the context, consider the nature of the verbal idea, decide upon the resultant meaning, and select the English idiom which will most nearly represent the meaning. Probably in no point have translators made more blunders than they have in rendering the aorist."
Bart Belteshassur said: "It is interesting to note that JW.Org changed the meaning of Luke 10:18." At that he said to them, ‘I began to behold Satan already fallen like lightening from heaven’ (old NWT) and ‘ ........I see Satan already fallen......from heaven.’" (new NWT). Both of these translations are wrong for the same reason and in this case the action of falling from heaven is complete not ongoing."
The ESV Study Bible comments on this verse: "It is not clear whether Jesus is speaking of a vision by which he saw something in the spiritual realm or if this simply a graphic declaration of what has been happening, but in either case Jesus indicates that Satan's authority and power over people has been decisively broken."
Although the imperfect tense used here ("I was watching Satan fall") is normally associated with the past, context has to be taken into consideration for the final rendering choice, as the ESV Committee suggested that the passage was not all that clear. Allowing the possibility of a vision here can justify the NWT translation reading. Jesus' reply that he saw Satan fall has been viewed several ways. One explanation of various given, is that Jesus may have been saying he saw the casting out of demons by the Seventy as the beginning of the ultimate defeat of Satan himself.
One use of the imperfect was used by the early editions of the NWT, where, in relation to the past, placed emphasis in the beginning of the action (which grammarians call - ingressive, inceptive). One feature of the imperfect denotes an incomplete action, as ongoing, that is, it excludes the assertion that the end of the action was attained. This does not mean that the NWT is correct in their interpretation, but they present one option of various explanations. In Greek, the principal tense to express completeness is the perfect tense. This is the reason that a perfect was not used at John 8.58, rather a Greek present tense was used to indicate Jesus' existence from an undefined past to the present ("I have been"). Hence, the action was not completed at the time of Jesus speaking, it was still going on.
We should keep in mind that the Greek language does not place emphasis on temporal significance, but in the kind of action. This is its fundamental significance.