Gospel Guidebook: Getting and Keeping It Right 
Misunderstandings about the Greek Present Tense
Many interpreters of the Bible stress the "durative" aspect of the present tense. They'll point to a passage like John 3:16 and say, "the word 'believe' is a present participle, and therefore it means 'continued belief,' and should be interpreted as the present continuous." They'll say that John 3:16 means something like "he who is believing [to the end of his life]." However, this type of interpretation is faulty. For starters, the present continuous is a tense that is used of something that has already started in the past. A thing cannot start and continue at the same time. So the question is: "When did they begin to believe?" One second ago, one year ago, a lifetime ago? How long exactly does a person need to believe to receive eternal life?
So, what is implied, if anything, by "durative" action? In Section 1852 of Herbert Weir Smyth's "A Greek Grammar for Colleges," he says, "Continued action is incomplete: hence nothing is stated as to the conclusion. Thus φεύγει he flees does not state whether or not the subject succeeded in escaping." In the case of the verb "believe" in John 3:16, even if we grant the questionable assumption that it is an action verb, as opposed to a stative verb, "he who is believing" could be as short as two milliseconds and still fulfill the requirements of "durative" action. First millisecond...continuing...second millisecond...end. There is nothing in the present participle of "believe" that implies that a person has continuous belief their whole life.
And why exactly is the present tense characterized as durative anyway? In Section 70 of D. B. Monro's "Homeric Grammar," he says the following: "The present is not a space of time, but a point; what is present therefore is not (generally speaking) a whole action or event, but the fact that it is in course of happening. So in English we usually say, not I write now, but I am writing now. The mere effort of regarding an action as in present time almost obliges us to give it a progressive character." This last sentence is profound. In other words, the past and the future are spaces of time, but the present is simply a point of time, so we are basically obliged to refer to it as durative action. But this "durative action" is not the type of "durative action" implied by so many interpreters who want to read their theology into the rules of Greek grammar.
Finally, is the the present tense always "durative"? On page 866 of A. T. Robertson's "A Grammar of the Greek New Testament in the Light of Historical Research," he says, "The Gnomic Present. This is the aorist present that is timeless in reality, true of all time." And this is exactly how the present participle is working in John 3:16 with the gnomic idea being signified by the Greek adjective pas (all, every) in the expression "everyone who believes."