Identify the error: He do not understand the problem.

The correct answer is do not because it violates the foundational rules of singular subject-verb agreement.

• Step 1 (Grammatical Rule): The pronoun 'He' is a third-person singular subject. In the present tense, third-person singular subjects must be paired with the singular auxiliary verb 'does', not the plural 'do'.


• Step 2 (Correction): To fix the error, the phrase must be adjusted to 'He does not understand the problem' (or the contraction 'doesn't').


• Incorrect Options:
  
  
  
  • He is incorrect because it is the correct third-person singular pronoun to introduce the sentence.
  
  
  • understand is incorrect because it is the correct base form infinitive that must follow the auxiliary helping verb.
  
  
  • the problem is incorrect because it serves as a perfectly valid direct object noun phrase to complete the predicate.