Isaiah 45:18 in Hebrew had the Tetragrammaton ( = YHWH) but the Greek Septuagint used ego eimi ( = i am), not Kyrios ( = Lord) as the substitute for the Tetragrammaton.
Οὕτως λέγει Κύριος ὁ ποιήσας τὸν οὐρανόν, οὗτος ὁ θεὸς ὁ καταδείξας τὴν γῆν καὶ ποιήσας αὐτήν, αὐτὸς διώρισεν αὐτήν, οὐκ εἰς κενὸν ἐποίησεν, ἀλλὰ κατοικεῖσθαι ἔπλασεν αὐτήν, Ἐγώ εἰμι, καὶ οὐκ ἔστιν ἔτι. (Isaiah 45:18 LXX)
It's highly likely that ego eimi in John 8:58 also represents the Tetragrammaton because it occured in the context where Jesus was claiming to be eternal.
The grammar of Psalm 90:2 in the Greek translation (the Septuagint) is the same as in John 8:58 and only differing in the inflection of the prounoun.
Psalm 90:2 (LXX) says that "before the mountains were...You are".
John 8:58: "before" Abraham was, I am".
The Jews correctly understood Jesus to be claiming pre-existent in vv. 56-57 but Jesus himself added more in v. 58. In this verse, Jesus also claimed to have existed in the same way Yahweh existed i.e. eternally. This is the reason why the Jews wanted to stone Jesus in v. 59. It's really the same reason found in John 10:33 which had a different scenario but the same claim of "blasphemy", because Jesus, "being a man, made himself God".
The present tense indicated that whether the mountains came into existence or not, Yahweh is deemed to be existing, to be always there existing, in contrast to the mountains which clearly had come into existence. Likewise, Christ's eternal existence was contrasted against the "coming of Abraham" in 3000 BCE:
Yahweh always exists whilst the mountains came into existence
Jesus always exists whilst Abraham came into existence
