John Gill's Bible Commentary Ver. 1. These also [are] the generations of Aaron and Moses , etc.] The descendants of them, those of the former, who is named first, because the eldest, were priests, and those of the latter Levites, and who are not very plainly pointed at, but are included among the Amramites, ( Numbers 3:27); the posterity of Moses being very obscure, only Levites, and these not particularly named but swallowed up among the Kohathites: find the following account was as it stood, in the day [that] the Lord spoke with Moses in mount Sinai ; and not, altogether as it then, was when he spoke to him in the wilderness, of Sinai, for then Aaron had four sons, but now two of them were dead as is after observed; and it seems to be for the sake of this circumstance chiefly that this clause is so put.
Matthew Henry Commentary
Verses 1-13 - There was much work belonging to the priests' office, and there wer now only Aaron and his two sons to do it; God appoints the Levites to attend them. Those whom God finds work for, he will find help for. The Levites were taken instead of the first-born. When He that made us saves us, as the first-born of Israel were saved, we are laid unde further obligations to serve him faithfully. God's right to us by redemption, confirms the right he has to us by creation.
Original Hebrew ואלה428 תולדת8435 אהרן175 ומשׁה4872 ביום3117 דבר1696 יהוה3068 את854 משׁה4872 בהר2022 סיני׃5514