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.