In this sentence, Casey used the double negative construction, "didn't know nothing," which is a characteristic feature of African American Vernacular English (AAVE) or some other nonstandard dialects.
The sentence "Casey didn't know nothing about the broken window" contains a double negative, which is a common feature of nonstandard English. Double negatives such as "didn't know nothing" are often used in some dialects or informal speech to emphasize a negative statement, although they are considered grammatically incorrect in standard English. In this case, the correct form would be "Casey didn't know anything about the broken window" or simply "Casey didn't know about the broken window."
