Short answers, pulled from the story.
Nader Shah began his invasion of India in May 1738 and entered Delhi on the 20th of March 1739. The city was sacked for several days following the massacre of the 22nd of March 1739, and Nader's forces departed on the 16th of May 1739.
Iranologist Michael Axworthy argues that Nader needed a breathing space for Persia to recover and a new source of cash to pay his army before renewing war against the Ottomans. Laurence Lockhart noted that continuous campaigns had caused famine in Persia and brought it close to bankruptcy, making India's wealth a necessary target.
It has been estimated that approximately 20,000 to 30,000 men, women and children were killed during the qatl-e-aam on the 22nd of March 1739, in the course of about six hours. A representative of the Dutch East India Company present in Delhi reported that some 10,000 women and children were also taken as slaves.
Nader Shah seized the Peacock Throne, the Koh-i-Noor diamond (Mountain of Light) and the Darya-i-Noor diamond (Sea of Light), along with other jewels from the Imperial Treasury. He also levied a fine of 20 million rupees on the people of Delhi. The Koh-i-Noor is now part of the British crown jewels and the Darya-i-Noor is part of the Iranian National Jewels.
The loss of the Mughal treasury dealt the final blow to the effective power of the Mughal Empire in India. The empire had already been fragmenting since the death of Emperor Aurangzeb in 1707, and the sack of Delhi accelerated its collapse. According to Michael Axworthy, the invasion also alerted the East India Company to the extreme weakness of the Mughals and the opportunity to fill the resulting power vacuum.
The qatl-e-aam was a massacre of Delhi's civilian population ordered by Nader Shah on the 22nd of March 1739 after 3,000 of his soldiers were killed in riots the previous night. The term means the killing of the common public in the open. Nader signalled the start by drawing his sword at the Sunehri Masjid and ended it by sheathing the sword after many hours of pleading by Muhammad Shah.