Complex sentence: Present-day El Salvador was inhabited by numerous Mesoamerican nations until their conquest by the Spanish Empire in 1525 forming part of the colony of New Spain until 1821, when it achieved independence as part of the Federal Republic of Central America.
Split into simpler sentences: Present-day El Salvador was inhabited by numerous Mesoamerican nations prior to the Spanish Empire's conquest in 1525. It remained part of the colony of New Spain until 1821, when it achieved independence as part of the Federal Republic of Central America.