שמות ז:יד פירוש: רש״י, רשב״ם, ספורנו ואור החיים

כבד — This word should be translated in the Targum by יקיר (his heart is hard — an adjective) and not by אתיקר (his heart has become Hard — a verb, as is given in some editions of Onkelos) because it is an adjective, just as (Exodus 18:18) “the thing is too heavy (כָּבֵד) for thee”.

כבד, Pharaoh had hardened his heart, as I explained already. (the word is a verb, not an adjective, compare Exodus 1,8 on the word מלא). In intransitive words applying to the body of a person, such as vayishman, he waxed fat, (Deut. 32,9) or vayichbad, he remained heavy, (Exodus 9,7) and several similar examples, constructed by a future tense with the prefix ו, the ordinary past tense would be kaved, or shamen.

כבד לב פרעה, even though Pharaoh could not help but notice the qualitative difference between what the sorcerers had done and what Moses and Aaron had accomplished.