The teacher explained the exercise in great detail.
Pronunciation: O DHA-ska-los ex-EE-yee-se tin A-skee-see po-LEE a-na-lee-tee-KA.
Grammar: «δάσκαλος» is a masculine noun; sg.: ο δάσκαλος, pl.: οι δάσκαλοι. Here it is in the nominative with the definite article «ο» as the subject of the verb «εξήγησε».