Why did HaShem have to introduce Himself to the Israelites in Mitzrayim?

Question

In Shemot 3:6 HaShem introduces Himself to Moshe and says: 'I am Elohei Avicha, Elohei Avraham, Elohei Yitzchak and Elohei Ya'akov'. In verse 13, Moshe asks HaShem which name he should tell the people and HaShem replies: 'Ehyeh Asher Ehyeh,' and tells him he should tell them that He is Ehyeh, and that He is HaShem, Elohei Avraham, Elohei Yitzchak and Elohei Ya'akov, and that this is His name for eternity from generation to generation. And even to the Ziknei Israel Moshe has to say that He is HaShem Elohei Avoteichem, Elohei Avraham, Yitzchak v'Ya'akov. Eventually Moshe has to tell Pharao that He is HaShem Elohei HaIvriyim. And to the Israelites Moshe has to show different signs in order for them to believe that He is indeed HaShem Elohei Avotam, Elohei Avraham, Elohei Yitzchak v'Elohei Ya'akov.

In Shemot 4:31 it says that the people believed that He was HaShem. But after Moshe visit to Pharao he gave the people harsh labour and then something changed..

1. Why is it that Moshe has to introduce HaShem? Didn't the people know who HaShem was?
2. And why does it seems like Moshe has to reintroduce HaShem to the people in Shemot 6:6-7?
3. And last but not least, when was it that the people knew HaShem?

Answer 1

Question 1) Bnei Yisrael worshiped Avodah Zarah in Egypt; see Yechezkel 20 and Idolatry in Mitzrayim? Thus, Moshe would have needed to remind his brethren that the G-d of their forefathers still cared about them.

Question 2) As stated in Shemot 6:2-3:

וַיְדַבֵּ֥ר אֱלֹהִ֖ים אֶל־מֹשֶׁ֑ה וַיֹּ֥אמֶר אֵלָ֖יו אֲנִ֥י יְהוָֽה׃ וָאֵרָ֗א אֶל־אַבְרָהָ֛ם אֶל־יִצְחָ֥ק וְאֶֽל־יַעֲקֹ֖ב בְּאֵ֣ל שַׁדָּ֑י וּשְׁמִ֣י יְהוָ֔ה לֹ֥א נוֹדַ֖עְתִּי לָהֶֽם׃

And God spoke unto Moses, and said unto him: ‘I am the LORD; and I appeared unto Abraham, unto Isaac, and unto Jacob, as God Almighty, but by My name (Yud-Kei-Vav-Kei) I made Me not known to them (Translation from Sefaria).

The re-introduction is for Hashem to reveal his Shem HaVayah, which connotates the keeping of past promises:

ושמי ה' לא נודעתי להם... שעליה נקרא שמי ה' נאמן לאמת דברי

And I did not reveal my Yud-Kei-Vav-Kei name to them... For that, my name was called Yud-Kei-Vav-Kei- faithful to the truth of my words.

Question 3) The people always knew Hashem. They may have neglected Him, or ignored Him, but they easily turned back to the Aibishter once Moshe arrived and performed the signs:

וַיְדַבֵּ֣ר אַהֲרֹ֔ן אֵ֚ת כָּל־הַדְּבָרִ֔ים אֲשֶׁר־דִּבֶּ֥ר יְהוָ֖ה אֶל־מֹשֶׁ֑ה וַיַּ֥עַשׂ הָאֹתֹ֖ת לְעֵינֵ֥י הָעָֽם׃ וַֽיַּאֲמֵ֖ן הָעָ֑ם וַֽיִּשְׁמְע֡וּ כִּֽי־פָקַ֨ד יְהוָ֜ה אֶת־בְּנֵ֣י יִשְׂרָאֵ֗ל וְכִ֤י רָאָה֙ אֶת־עָנְיָ֔ם וַֽיִּקְּד֖וּ וַיִּֽשְׁתַּחֲוּֽוּ׃

And Aaron spoke all the words which the LORD had spoken unto Moses, and did the signs in the sight of the people. And the people believed; and when they heard that the LORD had remembered the children of Israel, and that He had seen their affliction, then they bowed their heads and worshipped.

Answer 2

Chapter 3 of Exodus makes a lot more sense when you divide it according to the Documentary hypothesis. Dividing up the chapter into two sources, A Jahwist source an Elohist source, you actually get two separate stories. Thus, singling out the J passages in yellow, and the E passages in blue*, you can cleary read each colored text as a completely independent story.

The J source shows an all powerful known God, the god of Abraham that is already promising the future conquest, with Moses nearly absent. The E passage shows a very human Moses, with a unknown God, and unnamed forefathers.

So, to answer your questions, In source J, They people of Israel already know who YHWH is, as the God of Abraham. Therefore, Moses just needs to prove it was YHWH who actually sent him, as evidenced by his questions. In source E, The people of Israel don't know about God yet, and Moses expects them to ask about the name of the particular god that sent him.

*This division is my own, and is just a sample division which for simplicity ignores any other later sources (EJ and R). the exact division has been subject to various academic debates, e.g. in this paper from 1921.