NASB: “Jesus said to them, ‘Truly, truly, I say to you, before Abraham was born I am’” (as in most trans.)  

NWT: “Jesus said to them: ‘Most truly I say to you, Before Abraham came into existence, I have been.’”

Greek: εἶπεν αὐτοῖς Ἰησοῦς· ἀμὴν ἀμὴν λέγω ὑμῖν, πρὶν Ἀβραὰμ γενέσθαι ἐγὼ εἰμί (eipen autois Iēsous: amēn amēn legō humin, prin Abraam genesthai egō eimi).

That Jesus Christ, the Son of God, is truly God (YHWH) and truly man, is a main theme in John’s literature (esp. seen in the response of the Jews in John 5:17-18; 8:58-59; 10:30-33). The rejection of the person and nature of the Son as truly God (and man) by unitarian groups such as JWs (and Muslims, Oneness Pentecostals et al.) is chiefly based on a unitarian (or unipersonal) pre-committed theology that God is one sole person (unipernal), and not the exegesis of the text.   

Jesus declared in John 8:24: “For if you should not believe that I am [egō eimi] you will perish in your sins” (lit. trans.). The unpredicated egō eimi (“I am”) affirmation of the Son were unequivocally claims of deity–that is, claims of being truly God (and truly man)- esp. in light of YHWH’s exclusive claims of being the unpredicated egō eimi (“I am”) in the LXX (Deut. 32:39; Isa. 41:4; 43:10; 46:4).  

See Jesus’ Ἐγώ εἰμι, Egō Eimi (“I Am”) Declarations- John 8:58 for a detailed presentation of Jesus “I Am” claims.   
 

Exegetical Refutation: The Watchtower Bible and Tract Society’s (JWs) attempt to
‘Reason from the Scriptures’ regarding  John 8:58    

So strong was Jesus’ affirmation of deity in John 8:58 that the JWs’ Bible the NWT mistranslated the present active indicative verb, eimi (“am”) turning it into a past tense: “I have been.” From this, the JW’s argue that Jesus was not claiming to be deity (“I am”), but rather He was claiming to be “older” than Abraham was (as Michael the archangel), which incited the Jews to want to kill Him. However, what immediately refutes this false notion is:

1) Simply, the Greek text contains the present indicative verb eimi (“am”) and not any kind of past tense. In 1969, WT published a Greek Interlinear called, The Kingdom Interlinear Translation of the Greek Scriptures (KIT) and a revised ed. in 1985. The KIT is a Greek NT with English equivalents under each Greek word and the NWT on the side margins. What is interesting is that the Greek is unchanged, only the NWT is altered from the Greek.

For example, notice the photocopy of John 8:58-59 from the KIT in which you can see the unaltered Greek phrase egō eimi (“I am”) and the NWT’s altered reading “I have been,” on the side:

This clearly shows that the NWT purposely altered the Greek NT text, from the present “I am” to a past “I have been” (cf. Watchtower, 1 Sept. 1974, 526-27); as if Jesus was merely saying that He is older than Abraham-  only to fit the distinctive theology of the WT.

2) Even more, throughout the years, the WT has offered at least three reasons as to why the present tense verb eimi (“am”) should be translated as a past action (“have been”). First, in the 1950 ed. of the NWT, there is a footnote referring to the “I have been” rendering, which states: “I have been— ἐγώ εἰμι [egō eimi] . . . properly rendered in the perfect indefinite tense. . . .” (p. 312).

This sounds legitimate to one who is not familiar with Greek, however, there is no such tense as a “perfect indefinite” in biblical Greek. The WT made up a phony tense. Some have defended the WT’s explanation saying that “perfect indefinite” refers to the English, not the Greek. But we are not aware of a single official WT source that states this.

Then, the WT argued that the verb eimi was a “perfect indicative” (KIT, 1985 ed. 451). Now, there is a perfect indicative in Greek, however, the verb eimi takes no such form. And currently, the WT asserts that eimi is a “historical present”[9] explaining that “The verb ei·mi’, at John 8:58, is evidently in the historical present, as Jesus was speaking about himself in relation to Abraham’s past” Reasoning from the Scriptures, 418). Thus, the JWs see Jesus as merely claiming that He pre-existed Abraham, which, according to the JWs, enraged the Jews to the point of wanting to kill Him (cf. v. 59). This assertion, however, is flawed both grammatically and contextually.

First, a historical present tense occurs primarily in narrative literature and only in third person. In this context, Jesus was arguing with the Jews—He was not narrating. Secondly, the equative verb eimi is not used as a historical present. As the recognized Greek scholar, Daniel Wallace, points out, “The historical present is used fairly frequently in narrative literature to describe a past event” (Wallace, GGBB, 526).. Wallace also comments: 

If this is a historical present, it is apparently the only historical present in the NT that uses the equative verb eimi. The burden of proof, therefore, lies with the one who sees eimi as ever being used as a historical present. . . If this is a historical present, it is apparently the only historical present in the NT that is in other than third person (ibid., 530). 

The weight against the historical present view is massive. The reason for these various assertions of eimi postulated by the WT throughout the years (viz. the phony perfect indefinite; perfect indicative, and historical present) is, of course, obvious. If Jesus’ divine statements of being the “I am,” stand unmodified, then, Jesus made some astonishing and unambiguous claims of being the eternal God (as with John 5:17 and 10:27-30), which clearly show the WT to be a false religion in need of salvation.