Jesus said to them, Verily, verily, I say unto you, Before Abraham was, I am (John 8:58)
John 8:58 is a strong witness to the personal pre-existence of Jesus. On the phrase πρὶν Ἀβραὰμ γενέσθαι ἐγὼ εἰμι (“Before Abraham was, I am”) the Greek grammar necessitates that Jesus personally, not merely notionally, pre-existed. As one grammarian wrote:
Extension from Past. When used with an expression of either past time or extent of time with past implications (but not in past narrative), the present tense signals an activity begun in the past and continuing to present time: Lu 13:7 ιδου τρια ετη αφ ου ερχομαι ζητων καρπον . . . και ουχ ευρισκω, it is now three years since I have been coming looking for fruit . . .and not finding it; Lu 15:29 τοσαυτα ετη δουλευω σοι, I have been slaving for you all these years; Jn 14:9 τοσουτον χρονον μεθ υμων ειμι . . .; have I been with you so long . . .?; Ac 27:33 τεσσαρεσκαιδεκατην σημερον ημεραν προσδοκωντες ασιτοι διατελειτε, today is the fourteenth day you have been continuing on the alert without food; Jn 8:58 πριν Αβρααμ γενεσθαι εγω ειμι, I have been in existence since before Abraham was born. This is a form of the continuation realisation of the imperfective aspect, and similar uses are found with the imperfect tense and with imperfective participles. (K.L. McKay, A New Syntax of the Verb in New Testament Greek: An Aspectual Approach [Studies in Biblical Greek; New York: Peter Lang, 1994], 41-42, emphasis in bold added.)
Gary Michuta offered the following which, if correct, further supports the thesis that this verse supports the personal pre-existence of Jesus:
At first glance, it would have been Jesus' use of "I AM." As you know, God told Moses that His name is "I AM WHO AM." Is Jesus applying the divine name to himself? It seems so. However, that doesn't make sense in the context of the conversation. Jesus says Abraham "rejoiced to see my day." The Jews responded that He is not even 50 years old. Jesus responds, "Before Abraham came to be . . ." the divine name. It doesn't make a lot of sense. Moreover, how is it that Abraham rejoiced to see Jesus' day? There's more going on than meets the eye.
One possible answer might be found in Abraham's mysterious visit in Genesis 18. One day, three men came to Abraham, who shows hospitability and pays them reverence. One of the mysterious visitors says, "I will surely return to you about this time next year, and Sarah will have a son" (Genesis 18:10). Sarah, Abraham's wife, laughs because they are too old. The visitor answers, "Is anything too marvelous for the LORD to do? At the appointed time . . . next year, I will return to you, and Sarah will have a son" (Genesis 18:14). One year later, Sarah gave birth to Isaac, which means "laughter." The Scripture says, "The LORD took note of Sarah as he had said he would; he did for her as he had promised. Sarah . . . bore Abraham a son in his old age, at the set time that God had stated" (Genesis 21:1-2). Was this the day, the birth of his son "laughter," that Abraham rejoiced?
What's curious is that the text is not at all clear who or what was this mysterious visitor. The chapter begins by saying the Lord [Yahweh] appeared to Abraham, and looking up he saw three men (Genesis 18:1). After overhearing one of the visitors say that she will give birth, Sarah laughed. In response, the text says, "But the LORD [Yahweh] said to Abraham: 'Why did Sarah laugh . . . Is anything too marvelous for the LORD to do? At the appointed time, about this time next year, I will return to you, and Sarah will have a son" (Genesis 18:13-14). Also, Genesis 18:22 saying, "Then the men turned away from there and went toward Sodom, while Abraham was still standing before the Lord [Yahweh]." But there were three "men" who visited Abraham (Genesis 18:2) and only two angels came to Sodom (Genesis 19:1). Where was the third "man"? Apparently, the third "man" was the one still talking to Abraham, the Lord (Genesis 18:22). Our English translations say "Lord" in Genesis 18, but the Hebrew text gives the divine name YHWH ("I AM WHO AM").
Let's rewind back to our original passage in John 8. Jesus said Abraham "rejoiced to see my day." If this is the day of the miraculous birth of Isaac, then "my day" means the day of the mysterious visitor's return to Abraham and Sarah, who is YHWH ("I AM WHO AM"). If Abraham's visitation was a theophany (an appearance by God) then Jesus' application of the divine name fits perfectly with the whole discussion on Abraham. He visited Abraham. He is "I AM." It also explains why the Jews wanted to stone Jesus. (Gary Michuta, Behind the Bible: What The Bible Assumes You Already Know [Livona, Mich.: Nikaria Press, 2017], 169-71)