In the following question, a sentence is divided into numbered parts. Identify the part that contains an error in subject verb agreement. Sentence: “When a student works on a mathematical problem, he comes to a stage beyond which he do not know how to proceed, and where he is in doubt and perplexity.”

Introduction / Context:
This error spotting question focuses on subject verb agreement within a sentence about a student solving a mathematical problem. The sentence is split into parts. The phrase “he do not know” in part (2) appears suspect and must be checked against the basic rule that third person singular subjects require “does not” instead of “do not.”

Given Data / Assumptions:

• The full sentence is: “When a student works on a mathematical problem, he comes to a stage beyond which he do not know how to proceed, and where he is in doubt and perplexity.”
• Part (1) typically contains “When a student works on a mathematical problem, he comes to a stage beyond which.”
• Part (2) contains “he do not know how to proceed.”
• Part (3) contains “and where he is in doubt and perplexity.”
• The pronoun “he” is third person singular and normally takes “does not,” not “do not,” in the simple present tense.

Concept / Approach:
The main grammatical concept is that the auxiliary verb “do” must agree in number with the subject. For “he,” “she,” and “it,” the correct form is “does,” so the negative construction becomes “does not know.” The plural form “do not know” is used with “I,” “we,” “you,” and “they.” Recognising this subject verb mismatch allows us to pinpoint the error in the sentence.

Step-by-Step Solution:

Verification / Alternative check:
Rewrite the sentence correctly: “When a student works on a mathematical problem, he comes to a stage beyond which he does not know how to proceed, and where he is in doubt and perplexity.” This corrected version reads smoothly and obeys all subject verb agreement rules. Since only the auxiliary verb “do” needed to change to “does” in part (2), the location of the error is fully confirmed.

Why Other Options Are Wrong:

• Part (1): The structure “When a student works on a mathematical problem, he comes to a stage beyond which” is grammatically sound.
• Part (3): The clause “and where he is in doubt and perplexity” uses the correct form “is” and is error free.
• “No error”: This option cannot be correct because the phrase “he do not know” clearly violates subject verb agreement.

Common Pitfalls:
Students sometimes overlook auxiliary verbs when scanning for errors, focusing only on main verbs. Others may be influenced by colloquial speech where “do not” is used loosely with various subjects. For examination purposes, it is essential to stick to standard grammar rules and systematically test subject verb combinations, including auxiliary verbs, in every part of the sentence.

Final Answer:
Part (2) is the correct answer, because “he do not know” should be “he does not know.”