The phrase "hadst the impression" is an archaic version of "had the impression," and uses the second person singular form of "have" in the past tense, "hadst." The "-st" ending is a remnant of Old English verb conjugation, and represents the second person singular form. The pronunciation of "hadst" is /hædst/, with the "d" being pronounced as a voiced dental plosive. Overall, the spelling of "hadst the impression" reflects the historical development of the English language.