Complex sentence: In 1709, he participated in the expedition headed by Pedro de Aguirre, together with Fray Isidro de Espinosa, exploring the territory where now the city of San Antonio until Colorado River.
Split into simpler sentences: In 1709, he participated in the expedition headed by Pedro de Aguirre. Together, with Fray Isidro de Espinosa, they explored the territory from the modern-day city of San Antonio to the Colorado River.