"Jesus said to them, 'Very truly, I tell you, before Abraham was, I am.'” (Joh 8:58 NRSVue)
"Jesus said unto them, Verily, verily, I say unto you, Before Abraham was born, I am." (Joh 8:58 ASV)
Before Abraham was, I am] "I am that I am," in regard of mine eternal generation. And as mediator, "I am Jesus Christ, yesterday, today, and for ever," Hebrews 13:8 (John Trapp)
Before Abraham was, I am—The words rendered "was" and "am" are quite different. The one clause means, "Abraham was brought into being"; the other, "I exist." The statement therefore is not that Christ came into existence before Abraham did (as Arians affirm is the meaning), but that He never came into being at all, but existed before Abraham had a being; in other words, existed before creation, or eternally (as Joh 1:1). In that sense the Jews plainly understood Him, since "then took they up stones to cast at Him," just as they had before done when they saw that He made Himself equal with God (Joh 5:18). JFB
I am - The expression I am, though in the present tense, is clearly designed to refer to a past time. Thus, in Psa 90:2, “From everlasting to everlasting thou art God.” Applied to God, it denotes continued existence without respect to time, so far as he is concerned. We divide time into the past, the present, and the future. The expression, applied to God, denotes that he does not measure his existence in this manner, but that the word by which we express the present denotes his continued and unchanging existence. Hence, he assumes it as his name, “I AM,” and “I AM that I AM,” Exo 3:14. Compare Isa 44:6; Isa 47:8. There is a remarkable similarity between the expression employed by Jesus in this place and that used in Exodus to denote the name of God. The manner in which Jesus used it would strikingly suggest the application of the same language to God. The question here was about his pre-existence. The objection of the Jews was that he was not 50 years old, and could not, therefore, have seen Abraham. Jesus replied to that that he existed before Abraham. As in his human nature he was not yet 50 years old, and could not, as a man, have existed before Abraham, this declaration must be referred to another nature; and the passage proves that, while he was a man, he was also endowed with another nature existing before Abraham, and to which he applied the term (familiar to the Jews as expressive of the existence of God) I AM; and this declaration corresponds to the affirmation of John Joh 1:1, that he was in the beginning with God, and was God. Albert Barnes
58. ] As Lücke remarks, all unbiassed exegesis of these words must recognize in them a declaration of the essential præ-existence of Christ. All such interpretations of πρὶν Ἀβραὰμ γενέσθαι , as “ before Abraham became Abraham ,” i.e. father of many nations (Socinus and others), and of ἐγὼ εἰμί , as “ I was predetermined, promised by God ” (Grotius and the Socinian interpreters), are little better than dishonest quibbles . The distinction between γενέσθαι and εἰμί is important. “Antequam nasceretur Abraham, ego sum ” (Erasmus). The present εἰμί expresses essential existence , as in reff., especially Colossians 1:17 , and was often used by our Lord to assert His Divine Being. Alford's Greek NT
Jesus said unto them, Verily, verily, say unto you, Before Abraham was born, I am. The third occurrence of the solemn formula ‘Verily, verily,’ marks the highest point reached by the words of Jesus at this time. The substance of the words is in completest harmony with the form. In the clearest possible manner Jesus declares, not only His existence before Abraham, but also the essential distinction between His being and that of any man. Man is born, man passes through successive periods of time: of Himself, in regard alike to past, present, and future, Jesus says ‘I am.’ He claims for Himself that absolute, unchanging existence which is the attribute of God alone. If any argument be needed to enforce that which the words themselves supply, it is furnished in the conduct of the Jews (John 8:59), who clearly understood them to be a distinct (and in their mind a blasphemous) claim of that which belonged to God alone. The thought is distinctly present in the Old Testament: see Psalms 102:27, but especially Psalms 90:2. Schaff's Popular Commentary on the NT
The Jews understood what Jesus was saying to them:
"So they picked up stones to throw at him, but Jesus hid himself and went out of the temple." (Joh 8:59 NRSVue)