Complex sentence: In 1929, on the recommendation of Richard von Mises, he became Albert Einstein's assistant with the explicit understanding that he work with him on distant parallelism, and from 1931 to 1936, he collaborated with Albert Einstein on the theory of relativity.
Split into simpler sentences: In 1929, he became Albert Einstein's assistant with the explicit understanding that he work with him on distant parallelism. From 1931 to 1936, he collaborated with Albert Einstein on the theory of relativity.