Complex sentence: Johnson was contracted to finish the edition in 18 months but slowed on the work as the months passed; he told Charles Burney in December 1757 that it would take him until the following March to complete it.
Split into simpler sentences: Johnson was contracted to finish the edition in 18 months but as the months passed, his pace slowed. He told Charles Burney in December 1757 that it would take him until the following March to complete it.