Exodus 3:14 is very famous verse:
WTT: אֶהְיֶה אֲשֶׁר אֶהְיֶה
Which can be translated as I am who I am or I will be who I will be.
LXX: ἐγώ εἰμι ὁ ὤν
Because ὤν is participle (Present, Active, Nominative, Singular, Masculine) of εἰμι, This can be translated basically I am the being or I am the one who is.
Hebrew word היה is English word to be, and Greek word εἰμι. And the divine name, tetragrammaton—יהוה— is from that Hebrew verb.
1. Why did not LXX write something like “ἐγώ εἰμι ὁς ἐγώ εἰμι” ?
Because Hebrew sentence is so beautiful being perfect symmetry and the Greek not.
2. Why did not LXX use another word οὐσία? This is also present active participle of εἰμι. This word has kind of deeper meaning and loved by many philosophers which is often translated into English as substance or essence. So, the sentence would be like: “ἐγώ εἰμι ἡ οὐσία”
Greek professor simply said that the ancient people were sexists and did not want to use feminine noun for God. But 1 John 4:8 used feminine noun for God “ὁ θεὸς ἀγάπη ἐστίν” which is “God is love.”