China was easily defeated by the British in the first Opium War because the British had superior technology.
Step-by-step explanation:
The Chinese feudal society in time of the Opium war had digressed much from its original form as one of the leading innovators in the ancient and the medieval world.
They had not been updating their technology and the society had not changed for hundreds of years until the Europeans forced their entry into the country and changed their trade laws.
Thus, they had no experience in modern warfare and lost the war easily to the English force which knew and studied their weaknesses.