Complex sentence: On 4 August 1809, Captain Thomas Byam Martin of "Implacable", while off Hogland, assigned "Erebus" to patrol between Aspo and Sommars rock to harry Russian shipping and give warning should she spot the Russian fleet exiting Kronstadt.
Split into simpler sentences: On 4 August 1809, Captain Thomas Byam Martin of "Implacable", while off Hogland, assigned "Erebus" to patrol between Aspo and Sommars rock. Her mission was to harry Russian shipping and give warning should she spot the Russian fleet exiting Kronstadt.