Complex sentence: He remained in a military hospital until 15 April 1915, was promoted to "Oberleutnant" in May, and employed in the regimental replacement battalion as an instructor.
Split into simpler sentences: He remained in a military hospital until 15 April 1915, and was promoted to "Oberleutnant" in the following month. This was followed by a posting to the regimental replacement battalion as an instructor.