Complex sentence: Booth and Herold were sleeping at Garrett's farm on April 26 when soldiers from the 16th New York Cavalry arrived and surrounded the barn, announcing that they would set fire to it in fifteen minutes.
Split into simpler sentences: Booth and Herold remained at Garrett's farm until April 26, when Union soldiers from the 16th New York Cavalry arrived at the farm. The soldiers surrounded the barn, where Booth and Herold had been sleeping, and announced that they would set fire to it in fifteen minutes.