Short answers, pulled from the story.
Documents captured from the British steamer Automedon by the German raider Atlantis on the 11th of November 1940 revealed critical weaknesses of the Singapore base. These papers were passed to Japan and confirmed the true state of Fortress Singapore to Japanese intelligence officers by January 1941.
The Japanese 25th Army invaded Malaya on the 8th of December 1941 using bicycle infantry and light tanks to move swiftly through the jungle. They outflanked hastily established defensive lines and utilized superior close air support, armor, coordination, and experience to overwhelm Commonwealth forces.
Lieutenant-General Arthur Percival incorrectly guessed that the Japanese would land forces on the north-east side of Singapore and ignored advice that the north-west was a more likely direction of attack. He allocated much of the equipment and resources to the north-east sector while the depleted 8th Australian Division sector had no serious fixed defensive works.
Yamashita's heavy guns fired a bombardment of 88,000 shells that cut telephone lines and isolated forward units. No order was passed to the Commonwealth artillery units to conduct counter-battery fire, allowing about 13,000 men from 16 assault battalions to cross the Johor Strait and attack the 22nd Australian Brigade.
Percival formally surrendered shortly after 17:15 on the 15th of February 1942 at the Ford Motor Factory. Under the terms of the surrender, hostilities were to cease at 20:30 that evening, all military forces were to surrender unconditionally, and the British were allowed to maintain a force of 1,000 armed men to prevent looting.
Nearly 85,000 British, Indian, and Commonwealth troops were captured, in addition to losses during the earlier fighting in Malaya. About 5,000 men were killed or wounded, a majority of whom were Australian, while many prisoners died in Changi Prison or were transported to forced labor projects.