1. The adverb "hardly" is considered a negative.
2. According to the rules, modern English does not accept so-called double negatives.
a. I could not understand hardly anything. (two negatives)
b. I could understand hardly anything. ("Good" English)
I know hardly anything about your country. = "Good" English.
I don't know hardly anything about your country. = "Bad" English.
Student or Learner