John Calvin Commentary Exodus 10:21

"And Jehovah said unto Moses, Stretch out thy hand toward heaven, that there may be darkness over the land of Egypt, even darkness which may be felt." — Exodus 10:21 (ASV)

And the Lord said to Moses. God here inflicts the punishment without announcing it beforehand; because Pharaoh had deceitfully broken his promise of being obedient to His word. Since, therefore, he had so wickedly abused God’s clemency, he would inevitably be suddenly overtaken by a new calamity, so that he could in the darkness feel God’s avenging hand, which he had despised.

Nor, indeed, would he have been alarmed by threats; as it will soon appear that, when he was warned of the death of his firstborn, and of the same slaughter of both the firstborn of man and of beast throughout the whole land, he was unmoved, and in his complacency provoked God, as if he had heard nothing. It is no wonder, then, that God covered the whole land with darkness before Pharaoh could suspect anything of the kind.

At the end of the verse, some translate the word ימש,¹²⁵yamesh, passively; as if he had said that the darkness could be felt. For the word חשך, choshek, darkness,¹²⁶is singular in Hebrew. Those who take it transitively, because they suppose it to be put indefinitely, understand a noun, with this meaning, “that a man could feel.” But if the transitive sense is preferred, it will be better referred to Pharaoh. But I willingly subscribe to their opinion, who hold that the darkness was so thick that it could be felt by the hand.

¹²⁵ ימש, the vowels determine this verb to be in the Hiphil, or active causal voice. חשך, darkness, comes after the verb; the ordinary position of the nominative in Hebrew. The words, therefore, should naturally mean the darkness shall make (a man) feel. — W.

¹²⁶ Referring, of course, to the Latin plural noun tenebrae..