Complex sentence: Returning to the U.S., during the Second World War, Swadesh worked on military projects to compile reference materials on Burmese, Chinese, Russian, and Spanish and easy-to-learn textbooks for troops to learn Russian and Chinese.
Split into simpler sentences: Returning to the U.S., during the Second World War, Swadesh worked on military projects to compile reference materials on Burmese, Chinese, Russian, and Spanish. He also worked on easy-to-learn textbooks for troops to learn Russian and Chinese.