Complex sentence: The history of Tlingit is poorly known, mostly because there is no written record until first contact with Europeans around the 1790s, and even then it remains sparse and irregular until the early 20th century.
Split into simpler sentences: The history of Tlingit is poorly known, mostly because there is no written record until the first contact with Europeans around the 1790s. Documentation was sparse and irregular until the early 20th century.