Rashi on Exodus 8:26/רש"י על שמות ח׳:כ״ו

1ויעתר אל ה׳ AND ENTREATED THE LORD (the verb is the Kal) — he concentrated his energies on prayer. So, too, if it had wished to use the term וַיַעְתִּיר (the Hiphil) it could have used it, and that would have signified, “and he increased words in prayer”. But when it uses an expression denoting “and he did something” (the Kal) it signifies “he prayed much (devoutly)”.