Short answers, pulled from the story.
The Imperial German Navy commissioned the SM U-1 on the 14th of December 1906. This vessel featured a double hull and a single torpedo tube powered by an electric motor for submerged propulsion.
Schweffel and Howaldt constructed the Brandtaucher in Kiel but lost it during a test dive on the 1st of February 1851. Friedrich Krupp Germaniawerft later completed the Forelle in 1903 as the first fully functional German-built submarine.
Of 373 German U-boats built, 179 were operational or nearly operational at war end. A total of 1181 U-boats entered service before surrender with 785 lost during operations and 222 scuttled by crews.
The Treaty of Versailles signed at Paris Peace Conference in 1919 forbade construction of submarines entirely. It allowed only six battleships, six cruisers, twelve destroyers and twelve torpedo boats for Germany.
Germany restarted the unrestricted submarine campaign around the British Isles on the 1st of February 1917. This decision led to the US severing diplomatic relations on the 3rd of February and declaring war on Germany on the 6th of April.