Charles Ellicott Commentary Exodus 8:15

"But when Pharaoh saw that there was respite, he hardened his heart, and hearkened not unto them, as Jehovah had spoken." — Exodus 8:15 (ASV)

When Pharaoh saw that there was respite. —Hebrew, a breathing space.

He hardened his heart. —Until now Pharaoh’s nature had not been impressed; his heart had remained dull, callous, hard. Now an impression had been made (Exodus 8:8), and he must have yielded, if he had not called in his own will to efface it. In this was his great guilt. (See the comment on Exodus 4:21.)