Albert Barnes
The “fathers” are, as in Deuteronomy 4:37, the patriarchs, Abraham, Isaac, and Jacob. With them God did indeed make a covenant, but not the particular covenant now in question. The responsibilites of this later covenant, made at Sinai by the nation as a nation, attached in their day and generation to those whom Moses was addressing.
The “field” is added to the list of objects specifically forbidden in the parallel passage Exodus 20:17. The addition seems very natural in one who was speaking with the partition of Canaan among his hearers directly in view.
He added no more - i. e., He spoke no more with the great voice directly to the people, but addressed all other communications to them through Moses. This unique and sublime phenomenon, followed up by the inscription of the Ten Words on the two tables by the finger of God, marks not only the holiness of God’s Law in general, but the special eminence and permanent obligation of the Ten Words themselves as compared with the rest of the Mosaic enactments. The giving of the two tables did not take place until Moses had been on the Mount 40 days and 40 nights, as appears from the fuller account of Deuteronomy 9:9-12.
These verses contain a much fuller narrative of the events briefly described in Exodus 20:18-21. Here it is important to call attention to the fact that it was on the entreaties of the people that Moses had taken on him to be the channel of communication between God and them. God approved Deuteronomy 5:28 the request of the people, because it showed a feeling of their own unworthiness to enter into direct communion with God. The terrors of Sinai had done their work; they had awakened the consciousness of sin.