Complex sentence: From the 1830s until the 1940s, mechanical calculating machines such as adders and multipliers were built and improved, but they could not perform an "if/goto", and are not true computers.
Split into simpler sentences: From the 1830s to the 1940s, mechanical calculating machines such as adders and multipliers were built and improved. But these machines could not perform an "if/goto", and are not true computers.