Complex sentence: He attended Eton College and St John's College, Cambridge, and in 1774 when still only 20 was returned as MP for he family controlled borough of Stamford a seat he held until 1790.
Split into simpler sentences: He attended Eton College and St John's College, Cambridge. In 1774, when still only 20, he was returned as MP for the family controlled borough of Stamford, a seat he held until 1790.