Droanacharyaar- Drona the teacher

Guru Drona was the teacher of Pandavas and Kauravas and was the son of sage Bharadwaja. Durona and Dirubhada, the prince of the state of  Panchala, studied  together under the same Teacher or Guru and were good friends. As they were so close friends Dirubhada has promised to give half of his country to Drona once he ascended to the  throne.
After their completion of training they went away and Drona got married to Kripa’s sister. He had one son called 'Aswaththaama'. They were in abject poverty and Drona could not even feed them and  hence Drona wanted to get some help from Parasurama and went to him. But Parasurama had given away all his wealth by the time Drona went to him but had only the weapons and archery and other powers. So Dron took these and had gained the power to defeat anybody and became very powerful in all martial arts and war games.

Then  Drona  remembered his friend Dirubhada’s promise and  went to his bosom friend Dirubhada to ask for the promised half country. 

When Drona approached Dirubhada he was sitting as a king in audience with all his court members and when poor Drona went in his dirty and torn dress as a friend, he felt ashamed to call him as a friend.. He feigned ignorance and did not show any recognition as a friend or his promise. Drona tried to remind him about his promises but Dirubhada mocked him and insulted him and got him evicted from his court.

Drona was humiliated and came back to his brother-in-law Kirupa’s house at Hastinapura and stayed there. One day when the Pandavas and Kauravas were playing a ball game  the ball fell into a deep well and the boys could not retrieve the ball.  Drona who happened to pass by there saw this and  he  took the ball out by just sending an arrow into the well. The ball was brought up with an arrow. The boys were impressed by this feat and went and told  Bheeshma about it and Bheesham realized who the person could be and asked Drona  to teach these boys all his knowledge in archery and other weapon handling and other war games. 

Drona became Dronachchariyaar, the teacher.( Achchariyaar = Teacher)

So Drona taught them all he knew and once the princes learnt their lessons the boys asked Drona what they could do them in gratitude. Drona asked  The Kauravas to go and defeat the Dirubhada but they could not do it. Then Drona asked Arjuna to go and defeat Dirubhada and Arjuna defeated him and brought him as a prisoner. 

Drona  pardoned his friend Dirubhada and gave back his country to rule.

Thus insulted by Drona, Dirubhada  did penance and prayed to Lord Shiva to give his a son who would defeat and kill Drona and a daughter to marry Arjuna. God appeared and gave him the boon to have a son and daughter Drishtathumna and Draupati. The son could avenge for him and kill Drona. Drishthathumna achieved this under deceit.