<human>:  When did the British start trading opium with China?
<bot>: The British started trading opium with China in the 1840s. Opium was first imported into China from India in the 18th century, and it became a major part of the Chinese economy. In the 1840s, the British started to import opium from India to China, and they used it to pay for tea and other goods that they imported. The Chinese government didn't want to stop the opium trade, so they made it legal.