Take another look at the sentence:
Juan herd his mother calling from down the street but chose to ignore her summons. Although he knew that she would be angry about his late arrival for dinner, he had to finish his inspection of the dead toad on the sidewalk.
The first verb, herd, is the problem. The sentence requires the simple past. Heard is the correctly spelled form.
|Verb||Simple Present||Simple Past||Past Participle|
Knew, the verb you chose, is perfectly correct. Before you continue, review the rules for irregular verbs.