Complex sentence: The Inland North Dialect is the "standard Midwestern" speech that was the basis for General American in the mid- 20th Century, though it has been recently modified by the northern cities vowel shift.
Split into simpler sentences: The Inland North Dialect region was once considered the "standard Midwestern" speech that was the basis for General American in the mid- 20th Century. However, it has been recently modified by the Northern cities vowel shift.