Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
The Lord said - Or "the Lord had said." The first three verses of this chapter are parenthetical. Before Moses relates the last warning given to Pharaoh, he feels it right to recall to his readers' minds the revelation and command which had been previously given to him by the Lord.
When he shall let you go ... - When at last he lets you depart with children, flocks, herds, and all your possessions, he will compel you to depart in haste. Moses was already aware that the last plague would be followed by an immediate departure, and, therefore, measures had probably been taken to prepare the Israelites for the journey. In fact, on each occasion when Pharaoh relented for a season, immediate orders would of course be issued by Moses to the heads of the people, who were thus repeatedly brought into a state of more or less complete organization for the final movement.
Every man - In Exo 3:22 only women were named; the command is more explicit when the time has come for its execution.
Borrow - "ask." See Exo 3:22 note.
And Moses said - The following words must be read in immediate connection with the last verse of the preceding chapter.
About midnight - This marks the hour, but not the day, on which the visitation would take place. There may have been, and probably was, an interval of some days, during which preparations might be made both for the celebration of the Passover, and the departure of the Israelites.
Two points are to be noticed:
1. The extent of the visitation: the whole land suffers in the persons of its firstborn, not merely for the guilt of the sovereign, but for the actual participation of the people in the crime of infanticide Exo 1:22.
2. The limitation: Pharaoh's command had been to slay ALL the male children of the Israelites, but only one child in each Egyptian family was to die. If Tothmosis II was the Pharaoh, the visitation fell with special severity on his family. He left no son, but was succeeded by his widow.
The mill - This consisted of two circular stones, one fixed in the ground, the other turned by a handle. The work of grinding was extremely laborious, and performed by women of the lowest rank.
Firstborn of beasts - This visitation has a special force in reference to the worship of beasts, which was universal in Egypt; each district having its own sacred animal, adored as a manifestation or representative of the local tutelary deity.
Shall not a dog move his tongue - A proverb expressive of freedom from alarm and immunity front assault.