(א) וְאֵ֛לֶּה תּוֹלְדֹ֥ת אַהֲרֹ֖ן וּמֹשֶׁ֑ה בְּי֗וֹם דִּבֶּ֧ר ה' אֶת־מֹשֶׁ֖ה בְּהַ֥ר סִינָֽי׃ (ב) וְאֵ֛לֶּה שְׁמ֥וֹת בְּֽנֵי־אַהֲרֹ֖ן הַבְּכ֣וֹר ׀ נָדָ֑ב וַאֲבִיה֕וּא אֶלְעָזָ֖ר וְאִיתָמָֽר׃ (ג) אֵ֗לֶּה שְׁמוֹת֙ בְּנֵ֣י אַהֲרֹ֔ן הַכֹּהֲנִ֖ים הַמְּשֻׁחִ֑ים אֲשֶׁר־מִלֵּ֥א יָדָ֖ם לְכַהֵֽן׃
(1) Now these are the generations of Aaron and Moses in the day that God spoke with Moses in mount Sinai. (2) And these are the names of the sons of Aaron: Nadab the first-born, and Abihu, Eleazar, and Ithamar. (3) These are the names of the sons of Aaron, the priests that were anointed, whom he consecrated to minister in the priest’s office.
RASHI - These are the descendants of Moses and Aaron: Yet only the sons of Aaron are mentioned. However, they are considered descendants of Moses because he taught them Torah. This teaches us that whoever teaches Torah to the son of his fellow man, Our Sages regard it as if he had given life to him
...כל המלמד בן חבירו תורה מעלה עליו הכתוב כאילו ילדו
One who teaches a child, it is as if one had created that child.
Talmud Bavli 19b
(ד) וַיָּ֣מָת נָדָ֣ב וַאֲבִיה֣וּא לִפְנֵ֣י ה' בְּֽהַקְרִבָם֩ אֵ֨שׁ זָרָ֜ה לִפְנֵ֤י ה' בְּמִדְבַּ֣ר סִינַ֔י וּבָנִ֖ים לֹא־הָי֣וּ לָהֶ֑ם וַיְכַהֵ֤ן אֶלְעָזָר֙ וְאִ֣יתָמָ֔ר עַל־פְּנֵ֖י אַהֲרֹ֥ן אֲבִיהֶֽם׃ (פ)
(4) And Nadab and Abihu died before God, when they offered strange fire before God, in the wilderness of Sinai, and they had no children; and Eleazar and Ithamar ministered in the priest’s office in the presence of Aaron their father.
(א) וַיִּקְח֣וּ בְנֵֽי־אַ֠הֲרֹן נָדָ֨ב וַאֲבִיה֜וּא אִ֣ישׁ מַחְתָּת֗וֹ וַיִּתְּנ֤וּ בָהֵן֙ אֵ֔שׁ וַיָּשִׂ֥ימוּ עָלֶ֖יהָ קְטֹ֑רֶת וַיַּקְרִ֜בוּ לִפְנֵ֤י ה' אֵ֣שׁ זָרָ֔ה אֲשֶׁ֧ר לֹ֦א צִוָּ֖ה אֹתָֽם׃ (ב) וַתֵּ֥צֵא אֵ֛שׁ מִלִּפְנֵ֥י ה' וַתֹּ֣אכַל אוֹתָ֑ם וַיָּמֻ֖תוּ לִפְנֵ֥י ה'׃
(1) Nadav and Avihu, sons of Aharon, took their incense pans and put fire and incense in them, and offered an unauthorized fire before God, which He did not command them to do. (2) A fire went out from before God and burned them up, and they died before God.
Hazal gives many reasons for what happened to the two sons of Aaron. The more popular reasons are:
a) they acted arrogantly in the presence of Moshe
"The Torah recounts God's instructions to Moses at Mount Sinai in preparation for the giving of the Torah:
To Moses [God] said, 'Ascend to God, you, and Aaron, and Nadav and Avihu, and the seventy elders of Israel, and they shall prostrate themselves from afar ...' Moses and Aaron, Nadav and Avihu, and the seventy elders arose. They saw the Lord of Israel and beneath His feet, like a brickwork of sapphire ... And to the aristocracy of the Children of Israel, He [God] did not strike His hand. They viewed the Lord, they ate and drank. (Exodus 24:1,9-11)
This enigmatic passage may hold the key to understanding the offense of Nadav and Avihu. They are separated from the rest of the nation, leading them to think of themselves early on, at the time of the giving of the Torah, as future leaders. They are invited to join Moses, and they have a better vantage point than the rest of the nation. The purpose of the ascent is to bow from afar; instead, they stood and stared."
Aish HaTorah on Shimini
b) they performed an act that the Cohen HaGadol, their father was supposed to do,
c) they were drunk (a huge prohabition)
And God spoke to Aaron saying, 'Do not drink wine or strong drink, neither you nor your sons when you enter the Tent of Meeting lest you die.' (Leviticus 10:8-9)
d) they were not married and were not allowed to make this kind of offering.
FROM MORAH LEVIN: Why is are we reminded of this here? What is the point?
(ה) וַיְדַבֵּ֥ר ה' אֶל־מֹשֶׁ֥ה לֵּאמֹֽר׃ (ו) הַקְרֵב֙ אֶת־מַטֵּ֣ה לֵוִ֔י וְֽהַעֲמַדְתָּ֣ אֹת֔וֹ לִפְנֵ֖י אַהֲרֹ֣ן הַכֹּהֵ֑ן וְשֵׁרְת֖וּ אֹתֽוֹ׃ (ז) וְשָׁמְר֣וּ אֶת־מִשְׁמַרְתּ֗וֹ וְאֶת־מִשְׁמֶ֙רֶת֙ כָּל־הָ֣עֵדָ֔ה לִפְנֵ֖י אֹ֣הֶל מוֹעֵ֑ד לַעֲבֹ֖ד אֶת־עֲבֹדַ֥ת הַמִּשְׁכָּֽן׃ (ח) וְשָׁמְר֗וּ אֶֽת־כָּל־כְּלֵי֙ אֹ֣הֶל מוֹעֵ֔ד וְאֶת־מִשְׁמֶ֖רֶת בְּנֵ֣י יִשְׂרָאֵ֑ל לַעֲבֹ֖ד אֶת־עֲבֹדַ֥ת הַמִּשְׁכָּֽן׃ (ט) וְנָתַתָּה֙ אֶת־הַלְוִיִּ֔ם לְאַהֲרֹ֖ן וּלְבָנָ֑יו נְתוּנִ֨ם נְתוּנִ֥ם הֵ֙מָּה֙ ל֔וֹ מֵאֵ֖ת בְּנֵ֥י יִשְׂרָאֵֽל׃ (י) וְאֶת־אַהֲרֹ֤ן וְאֶת־בָּנָיו֙ תִּפְקֹ֔ד וְשָׁמְר֖וּ אֶת־כְּהֻנָּתָ֑ם וְהַזָּ֥ר הַקָּרֵ֖ב יוּמָֽת׃ (פ)
(5) And God spoke to Moshe, saying: (6) ’Bring the tribe of Levi near, and set them before Aaron the priest, that they may serve him. (7) And they shall keep guard, and the guard of the whole congregation before the tent of meeting, to do the service of the tabernacle. (8) And they shall keep all the furniture of the tent of meeting, and the preservation of the children of Israel, to do the service of the tabernacle. (9) And you will give the Levites to Aaron and to his sons; they are wholly given to him from the children of Israel. (10) And you shall appoint Aaron and his sons, that they may keep their priesthood; and the common man that comes near will be put to death.
RASHI that they may serve him: What form does this service take? “They shall keep his charge” (verse 7). Since guarding the Mishkan so that no stranger (non- kohen) should come near is his obligation, as it says, “You, your sons, and your father’s house with you, shall bear the iniquity of the Mishkan” (Num. 18:1), and these Levites assisted him this was the service.
You shall appoint Aaron…: [The word תִּפְקֹד is] an expression of appointment, not of counting.
The Rambam says "The task of the Leviim was not so much to protect the Mishkan [and Beis Hamikdash] as a militia, but rather to serve as an honor guard, as befits the royal palace."
(יא) וַיְדַבֵּ֥ר ה' אֶל־מֹשֶׁ֥ה לֵּאמֹֽר׃ (יב) וַאֲנִ֞י הִנֵּ֧ה לָקַ֣חְתִּי אֶת־הַלְוִיִּ֗ם מִתּוֹךְ֙ בְּנֵ֣י יִשְׂרָאֵ֔ל תַּ֧חַת כָּל־בְּכ֛וֹר פֶּ֥טֶר רֶ֖חֶם מִבְּנֵ֣י יִשְׂרָאֵ֑ל וְהָ֥יוּ לִ֖י הַלְוִיִּֽם׃ (יג) כִּ֣י לִי֮ כָּל־בְּכוֹר֒ בְּיוֹם֩ הַכֹּתִ֨י כָל־בְּכ֜וֹר בְּאֶ֣רֶץ מִצְרַ֗יִם הִקְדַּ֨שְׁתִּי לִ֤י כָל־בְּכוֹר֙ בְּיִשְׂרָאֵ֔ל מֵאָדָ֖ם עַד־בְּהֵמָ֑ה לִ֥י יִהְי֖וּ אֲנִ֥י ה' (ס)
(11) And the God spoke to Moshe, saying: (12) ’And I, behold, I have taken the Levites from among the children of Israel instead of every first-born that opens the womb among the children of Israel; and the Levites shall be Mine; (13) for all the first-born are Mine: on the day that I smote all the first-born in the land of Egypt I sanctified unto Me all the first-born in Israel, both man and beast, Mine they shall be: I am the God.’ .
RASHI from among the children of Israel: That the Israelites should have to hire them for My service? God gained His right to them through the Israelite firstborns, taking them [the Levites] in their place. For originally the service was performed by the firstborns, but when they sinned by worshipping the golden calf, they became disqualified. The Levites, who had not committed idolatry, were chosen in their stead. [Midrash Aggadah]
(יד) וַיְדַבֵּ֤ר ה' אֶל־מֹשֶׁ֔ה בְּמִדְבַּ֥ר סִינַ֖י לֵאמֹֽר׃ (טו) פְּקֹד֙ אֶת־בְּנֵ֣י לֵוִ֔י לְבֵ֥ית אֲבֹתָ֖ם לְמִשְׁפְּחֹתָ֑ם כָּל־זָכָ֛ר מִבֶּן־חֹ֥דֶשׁ וָמַ֖עְלָה תִּפְקְדֵֽם׃
(14) And the God spoke to Moses in the wilderness of Sinai, saying: (15) ’Number the children of Levi by their fathers’ houses, by their families; every male from a month old and upward shalt thou number them.’
CHAZAL - Why מִבֶּן־חֹ֥דֶשׁ וָמַ֖עְלָה and not after the age of 20 like everyone else? Although the Leviim officially served the Cohanim in the Mishkan after the age of 30, Hashem wanted to make sure that the parents were always aware that they were raising the future caretakers of the Mishkan & B'nai Yisrael. They should be raised with a level of Keduasha preparing them for a life of Kedusha
(לט) כָּל־פְּקוּדֵ֨י הַלְוִיִּ֜ם אֲשֶׁר֩ פָּקַ֨ד מֹשֶׁ֧ה וְׄאַׄהֲׄרֹ֛ׄןׄ עַל־פִּ֥י ה' לְמִשְׁפְּחֹתָ֑ם כָּל־זָכָר֙ מִבֶּן־חֹ֣דֶשׁ וָמַ֔עְלָה שְׁנַ֥יִם וְעֶשְׂרִ֖ים אָֽלֶף׃ (ס)
(39) All that were numbered of the Levites, whom Moses and Aaron numbered at the commandment of God by their families, all the males from a month old and upward, were twenty and two thousand.
(מ) וַיֹּ֨אמֶר ה' אֶל־מֹשֶׁ֗ה פְּקֹ֨ד כָּל־בְּכֹ֤ר זָכָר֙ לִבְנֵ֣י יִשְׂרָאֵ֔ל מִבֶּן־חֹ֖דֶשׁ וָמָ֑עְלָה וְשָׂ֕א אֵ֖ת מִסְפַּ֥ר שְׁמֹתָֽם׃ (מא) וְלָקַחְתָּ֨ אֶת־הַלְוִיִּ֥ם לִי֙ אֲנִ֣י ה' תַּ֥חַת כָּל־בְּכֹ֖ר בִּבְנֵ֣י יִשְׂרָאֵ֑ל וְאֵת֙ בֶּהֱמַ֣ת הַלְוִיִּ֔ם תַּ֣חַת כָּל־בְּכ֔וֹר בְּבֶהֱמַ֖ת בְּנֵ֥י יִשְׂרָאֵֽל׃ (מב) וַיִּפְקֹ֣ד מֹשֶׁ֔ה כַּאֲשֶׁ֛ר צִוָּ֥ה ה' אֹת֑וֹ אֶֽת־כָּל־בְּכֹ֖ר בִּבְנֵ֥י יִשְׂרָאֵֽל׃ (מג) וַיְהִי֩ כָל־בְּכ֨וֹר זָכָ֜ר בְּמִסְפַּ֥ר שֵׁמ֛וֹת מִבֶּן־חֹ֥דֶשׁ וָמַ֖עְלָה לִפְקֻדֵיהֶ֑ם שְׁנַ֤יִם וְעֶשְׂרִים֙ אֶ֔לֶף שְׁלֹשָׁ֥ה וְשִׁבְעִ֖ים וּמָאתָֽיִם׃ (פ)
(40) And God said to Moshe: ‘Number all the first-born males of the children of Israel from a month old and upward, and take the number of their names. (41) And thou shalt take the Levites for Me, even the God, instead of all the first-born among the children of Israel; and the cattle of the Levites instead of all the firstborn among the cattle of the children of Israel.’ (42) And Moshe numbered, as God commanded him, all the first-born among the children of Israel. (43) And all the first-born males according to the number of names, from a month old and upward, of those that were numbered of them, were twenty and two thousand two hundred and threescore and thirteen.
RAMBAM Previously all of the 1st born had a higher status. However, after the sin of the Golden Calf, where the 1st born misused their status, God took the elevated status away from them. The Leviim did not participate, so they retained the kavod.
(מד) וַיְדַבֵּ֥ר ה' אֶל־מֹשֶׁ֥ה לֵּאמֹֽר׃ (מה) קַ֣ח אֶת־הַלְוִיִּ֗ם תַּ֤חַת כָּל־בְּכוֹר֙ בִּבְנֵ֣י יִשְׂרָאֵ֔ל וְאֶת־בֶּהֱמַ֥ת הַלְוִיִּ֖ם תַּ֣חַת בְּהֶמְתָּ֑ם וְהָיוּ־לִ֥י הַלְוִיִּ֖ם אֲנִ֥י ה' (מו) וְאֵת֙ פְּדוּיֵ֣י הַשְּׁלֹשָׁ֔ה וְהַשִּׁבְעִ֖ים וְהַמָּאתָ֑יִם הָעֹֽדְפִים֙ עַל־הַלְוִיִּ֔ם מִבְּכ֖וֹר בְּנֵ֥י יִשְׂרָאֵֽל׃ (מז) וְלָקַחְתָּ֗ חֲמֵ֧שֶׁת חֲמֵ֛שֶׁת שְׁקָלִ֖ים לַגֻּלְגֹּ֑לֶת בְּשֶׁ֤קֶל הַקֹּ֙דֶשׁ֙ תִּקָּ֔ח עֶשְׂרִ֥ים גֵּרָ֖ה הַשָּֽׁקֶל׃ (מח) וְנָתַתָּ֣ה הַכֶּ֔סֶף לְאַהֲרֹ֖ן וּלְבָנָ֑יו פְּדוּיֵ֕י הָעֹדְפִ֖ים בָּהֶֽם׃ (מט) וַיִּקַּ֣ח מֹשֶׁ֔ה אֵ֖ת כֶּ֣סֶף הַפִּדְי֑וֹם מֵאֵת֙ הָעֹ֣דְפִ֔ים עַ֖ל פְּדוּיֵ֥י הַלְוִיִּֽם׃ (נ) מֵאֵ֗ת בְּכ֛וֹר בְּנֵ֥י יִשְׂרָאֵ֖ל לָקַ֣ח אֶת־הַכָּ֑סֶף חֲמִשָּׁ֨ה וְשִׁשִּׁ֜ים וּשְׁלֹ֥שׁ מֵא֛וֹת וָאֶ֖לֶף בְּשֶׁ֥קֶל הַקֹּֽדֶשׁ׃ (נא) וַיִּתֵּ֨ן מֹשֶׁ֜ה אֶת־כֶּ֧סֶף הַפְּדֻיִ֛ם לְאַהֲרֹ֥ן וּלְבָנָ֖יו עַל־פִּ֣י ה' כַּאֲשֶׁ֛ר צִוָּ֥ה ה' אֶת־מֹשֶֽׁה׃ (פ)
(44) And God spoke to Moshe, saying: (45) ’Take the Levites instead of all the first-born among the children of Israel, and the cattle of the Levites instead of their cattle; and the Levites shall be Mine, even the LORD’S. (46) And as for the redemption of the two hundred and three score and thirteen of the first-born of the children of Israel, that are over and above the number of the Levites, (47) thou shalt take five shekels apiece by the poll; after the shekel of the sanctuary shalt thou take them—the shekel is twenty gerahs. (48) And thou shalt give the money wherewith they that remain over of them are redeemed unto Aaron and to his sons.’ (49) And Moses took the redemption-money from them that were over and above them that were redeemed by the Levites; (50) from the first-born of the children of Israel took he the money: a thousand three hundred and threescore and five shekels, after the shekel of the sanctuary. (51) And Moses gave the redemption-money unto Aaron and to his sons, according to the word of the LORD, as the LORD commanded Moshe.
Moshe figured out that there were 22,273 1st born males in B"Y and there were a total of 22,000 Leviim. God told Moshe that B"Y would have to make up for this surplus. Each of the "extra" 273 bachorim had to donate five 1 shekel coins and this money was given to Aaron and his sons.
In order to avoid men saying that they were not one of the "extra" ones - Moshe made a lottery
FROM MORAH LEVIN: WHAT IS THE REASON FOR THIS UNBALANCE?
To even it out and to make sure that there would be enough to sustain Aaron and his sons, each of the extra 273 bachors were commanded to give five 1 shekel coins to be given to the Cohanim (Aaron & his sons). This would release these extra 273 men from Avoda in the future.