Complex sentence: He began training by night to use his weapons in absolute darkness, and steadily achieved a great level of skill.Droṇa was greatly impressed by Arjuna's concentration, determination and drive, and promised him that he would become the greatest archer on earth.
Split into simpler sentences: He began training by night to use his weapons in absolute darkness and steadily achieved a great level of skill. Drona was greatly impressed by Arjuna's concentration, determination and drive and promised him that he would become the greatest archer on earth.