I am sure. — Heb., I know, which is more suitable, since it is God who speaks, and to Him the future is known with as absolute a certainty as the past.
No, not by a mighty hand. — Rather, not even under a mighty hand (ne quidem valida manu castigatus, Rosenmüller). Pharaoh, even when chastised by My mighty hand, will not voluntarily permit of your departure (see Exodus 14:5-23).