Complex sentence: Modern-day Lagos was founded by Portugal in the sixteenth century, and was a major centre of the slave trade until 1851, when the United Kingdom, which had abolished slavery in 1807, captured the city.
Split into simpler sentences: Modern-day Lagos was founded by Portugal in the sixteenth century. It was a major centre of the slave trade until 1851, when the United Kingdom, which had abolished slavery in 1807, captured the city.