Ramban

AND THE ETERNAL PASSED BY BEFORE HIM. This means that He fulfilled His promise to Moses, I will make all My goodness pass before thee.
AND HE PROCLAIMED: ‘HASHEM HASHEM E-IL’ (THE ETERNAL, THE ETERNAL, G-D). These three words are sacred Names of G-d which the Sages call midoth (attributes), being that they constitute the attribute of the Lord of repentance, the attribute of His mercies and that of His goodness. as separate attributes. See, however, Guide of the Perplexed, Vol. I, p. 193, Note 5, in Friedlander’s translation. The Proper Name of G-d [i.e. the Tetragrammaton], however does not lend itself to any plural form.] does not lend itself to any plural form, in spite of the fact that the various attributes are here cited (Abusaula). And the attributes which are perceived in human terms are ten: merciful and gracious, etc. Thus on one side they are all attributes, and on the other, there are the three which denote the Names of His essence, whereas the Ten are attributes. Now the attributes also represent Names of G-d, for merciful and gracious, long-suffering etc. are all with reference to the essence of G-d the Most High. Therefore it does not say “the G-d Who has mercy, grace, and is long-suffering,” for [that would have meant that these are His ways with the lower creatures; instead it says, merciful and gracious, long-suffering, indicating] that these actions emanate from G-d’s attributes, [which are His essence].
And abounding in goodness and truth; keeping mercy upon the thousandth generation — these three denote the attribute of mercy, since He increases the goodness over His strength and might and the truth in His mercies. And ‘notzeir’ (keeping) mercy unto the thousandth generation, for He hath remembered His mercy and His faithfulness toward the house of Israel. Or it may be that the word notzeir means “sprouting,” of the root, ‘v’neitzer’ (and a twig) shall grow forth out of his roots. And in His goodness He is ‘nosei’ (forgiving) iniquity and transgression and sin. This is of the expression, I have made, and I ‘esa’ (will bear). The two phrases, and that will by no means clear the guilty, and visiting the iniquity of the fathers upon the children, are an explanation to the one of forgiving iniquity [mentioned before]. It is called an “attribute” because He clears the sinner by this visitation. And because this act of forgiving is not equal for iniquity, transgression and sin, but instead in each category has its own form of clearing the sinner, it is called in each case “one attribute.” etc., also constitute an independent attribute, for the ‘clearing’ done in this form of forgiving is by visitation, it being another way of forgiveness of sin, that He will not forgive it outright, but visit it upon the generations etc.” (Abusaula).
Now Moses bowed his head to G-d Who passed by before him, and prayed that G-d should always go in their midst and forgive their iniquity and their sin in whatever they might do, and that He cause them to inherit the Land. Moses, however, did not ask that G-d forgive “our transgressions,” since it is not possible that G-d forgive transgressions, which are sins of a rebellious nature; in those cases He is only to bear them [through visitation] and not to destroy them. It will not be concealed from you why these two Names [If now I have found grace in Thy sight, O Lord, let the Lord, I pray Thee, go in the midst of us…] are written with Aleph Daleth [instead of in the letters of the Tetragrammaton];. Thus the first Name is in place of the Tegragrammaton, and Moses prayed that the Lord [written in Aleph Daleth] go in our midst in the absolute perfection of the Unity of the two Names (Abusaula). it is for this reason that G-d said, Behold, I make a covenant, until [all people shall see…] the work of the Eternal that it is tremendous.