Why did Dronacharya believe that his son Ashwathama is dead

Dronacharya was Arjuna's teacher and an expert in the art of archery. Defeating him in battle was not easy. To achieve this, Krishna and Arjuna devised a strategy. They named an elephant "Ashwatthama" and arranged for Yudhishthira to announce that Ashwatthama had been killed in battle. In the heat of the battle, they pressed Yudhishthira to declare, `Ashwatthama hatah – naro va kunjarah`

Dronacharya only heard the part "Ashwatthama is dead," which caused him confusion, as he trusted Yudhishthira’s truthfulness. Doubtful and heartbroken, Dronacharya abandoned his weapons and stopped fighting. In this vulnerable moment, Drupada's son, Dhrishtadyumna, killed him to avenge his father.

Drona and Drupada were once fellow students. When Drupada became a king, Drona sought his friendship and assistance. However, Drupada scorned him, saying, "What friendship can exist between a king and a poor man like you?" Insulted, Drona trained Arjuna, making him a master archer. In revenge, he sent Arjuna to defeat Drupada, who was then humiliated and subdued.

Humiliated, Drupada performed a fire-sacrifice to beget a son capable of killing Drona. This sacrifice produced Dhrishtadyumna and Draupadi (also called Yajnaseni or Krishnaa), who would play pivotal roles in the Mahabharata.