Read each sentence to find out whether there is any grammatical error in it.…

Concept: The case of a relative pronoun -- who vs. whom -- is fixed by its grammatical role in its own clause: 'who' functions as a subject (it performs the verb's action), while 'whom' functions as an object (it receives the action of a verb or preposition). A short parenthetical comment such as 'I know', 'I think', or 'I believe', inserted between the pronoun and the verb that governs it, does not change the pronoun's case -- the verb that follows the parenthetical still decides it.

Application: In 'the teacher whom I know has helped my son', remove the parenthetical 'I know' to see the underlying structure: 'the teacher ___ has helped my son'. The pronoun is the subject of 'has helped' (the teacher is the one who helped), so the subject form 'who' is required here, not the object form 'whom'.

• 'She is the teacher' -- correctly introduces the subject with the linking verb 'is'; no error.

• 'has helped my son' -- the present-perfect verb form correctly agrees with its singular subject; the fault lies in the pronoun before it, not in this verb phrase.

• 'in the examination' -- a correctly formed prepositional phrase.

• 'whom I know' -- the error: the object pronoun 'whom' is wrongly used where the subject pronoun 'who' is required.

Result: The grammatical error is in the part 'whom I know'.