Greek and English tenses are different. Many problems of biblical interpretation stem from people trying to interpret the Bible with a modern understanding, and with an English understanding of how grammar works. A prime example of this would be John 8:58.
English tenses emphasize time - past, present, future. Greek tenses don't - they emphasize "aspect," such as ongoing continuous action, completed action, undefined action, etc.
The Greek Present Tense *usually* occurs in the present, but it sometimes occurs in the past or the future.
One function of the Greek Present Tense is the Extension From Past. Here is a discussion of this function from a couple of sources:
"The Present Of Past Action Still In Progress. The Present Indicative, accompanied by an adverbial expression denoting duration and referring to past time, is sometimes used in Greek, as in German, to describe an action which, beginning in past time, is still in progress at the time of speaking. English idiom requires the use of the Perfect in such cases." -- Burton's moods and tenses, Ernest Burton
"The present tense may be used to describe an action which, begun in the past, continues in the present. The emphasis is on the present time. Note that this is different from the perfect tense in that the perfect speaks only about the results existing in the present time. It is different from the progressive present in that it reaches back in time and usually has some sort of temporal indicator, such as an adverbial phrase, to show this past-referring element."
"The key to this usage is normally to translate the present tense as an English present perfect." -- Greek Grammar Beyond the Basics, Daniel Wallace, pages 519-520
John 8:58 fits the requirements:
1) The verb is in the Greek Present Tense -- ειμι (I am)
2) It has a temporal indicator, an adverbial phrase, to show a past-referring element -- πρὶν Ἀβραὰμ γενέσθαι (before Abraham was born)
John 15:27 has the same Greek Present Tense verb, but being 2nd person plural. The structure of the sentence matches that of John 8:58, yet the majority of English translations render it in the English Perfect, not in the English Present, just like the NWT in John 8:58.
For those who appeal to "context" to support "I AM" as meaning deity: the Jews' question was about his age, not his identity; the context supports the NWT's rendering. Some say Jews seeking to stone Jesus after John 8:58 is proof he was claiming to be God. But this is a logical fallacy. That is, "claiming to be God" would indeed fall under the category of "blasphemy" to the Jews, but it is not the only thing in that category. One doesn't have to look far in Scripture to find the Jews seeking to kill Jesus for other reasons. Hence, that reasoning is fallacious. Also, Jesus' words at John 8:58 didn't make the Jews decide to kill Jesus - they had already made that decision. Jesus himself said just a few verses earlier, in verse 37, that the Jews were seeking to kill him. So this line of reasoning is fallacious on two accounts.
One must keep in mind that the NWT was neither the first nor the last translation to see John 8:58 as the Greek Present Tense Extension From Past. The two translations of Moffatt and Goodspeed clearly pre-date the NWT, and therefore came to that conclusion before the NWT.
Some say there is "no perfect indefinite tense in Greek" - or something like that. However, neither have Jehovah' Witnesses. This is a misquote of what JW publications have said at different times about John 8:58. That is, JW publications have discussed the GREEK grammar, and at other times discussed the ENGLISH grammar - the tense of the English translation. The Greek is clearly in the Greek, but the English translation in the NWT is clearly in the Perfect. Yet some NWT critics will put quotes like these side by side, so as to "prove" that the NWT translators were contradictory and inconsistent.
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I just checked the original quote. It said "rendered in the perfect indefinite tense." Note it said "rendered in," not "rendered from." So this tense would be an English tense, not Greek.
There is indeed an English perfect indefinite tense, though it may be a term no longer in use:
http://tinyurl.com/4nznoeu
This is one way to render the Greek's Present Tense Extension From Past.