What is the incorrect part of the following sentence? "He don’t know how to fix the car."

The phrase "don’t" is considered incorrect in this sentence due to subject-verb agreement. The subject "He" is singular, and thus requires the singular form of the verb "does" for proper grammatical structure. The correct phrase would be "He doesn’t know how to fix the car." In standard English, "don’t" is used with plural subjects or with "I" and "you," while "doesn't" is the appropriate contraction for third-person singular subjects like "he," "she," or "it." This distinction is essential for maintaining grammatical accuracy in sentences.