Short answers, pulled from the story.
U-boat is an anglicized form of the German word U-Boot, itself a shortening of Unterseeboot, meaning undersea boat. The term was applied to German naval submarines and also, historically, to submarines of the Austro-Hungarian Navy.
The Forelle, completed by the Friedrich Krupp Germaniawerft dockyard in Kiel in 1903, was the first fully functional German-built submarine. Krupp sold it to Russia during the Russo-Japanese War in April 1903.
Germany's unrestricted submarine campaign in World War I failed primarily because the introduction of convoy escort in August 1917 sharply reduced shipping losses. The campaign also backfired diplomatically: the sinking of the RMS Lusitania and other incidents contributed to the United States declaring war on Germany on the 6th of April 1917.
Black May refers to May 1943, when U-boat losses in the Battle of the Atlantic began to outpace the damage U-boats inflicted on Allied shipping. On the 24th of May 1943, Karl Dönitz halted the convoy campaign because losses had become unsustainable.
Germany established the Ingenieurskantoor voor Scheepsbouw (IVS), a submarine design office run by Krupp and based in the Netherlands. IVS designed and built submarines for foreign navies, including the Vetehinen-class in Finland and a large boat for Spain, maintaining German expertise while circumventing the treaty ban. These secret programs were exposed in the Lohmann Affair, which forced the head of the Reichsmarine, Hans Zenker, to resign.
The Type XXI, known as the Elektroboot, was a World War II German submarine designed for greatly improved submerged speed and endurance using massively enlarged battery capacity. Its hull design derived from the experimental Walter hydrogen-peroxide submarine program; when the Walter propulsion system proved impractical, the large hull space was repurposed for batteries. The Type XXI was mass-produced in prefabricated sections, but only two set out on war patrols before Germany surrendered.