Leonhard Euler was a Swiss mathematician, physicist and astronomer, who was enormously prolific and influential in mathematics in the 18th century. Euler's influence spanned from numerous branches of mathematics to mechanics, astronomy, fluid dynamics and even musical theory.
The correct pronunciation of Euler's first name, Leonhard is Leh-ohn-hahrd, where the "r" is barely pronounced, similarly to how the "r" is pronounced in a British accent. His name, Euler, is often mispronounced as you-ler or even wheel-er. Fortunately, Euler is actually easy to pronounce. The correct pronunciation of Euler is Oil-ah. It must be noted that while there is an "r" in his name, the resulting sound, in German, is as if it were barely present. Compared to Leonhard, the "r" in Euler is basically silent, so the "er" at the end of his surname produces something similar to an "ah" sound.