Grammar Rules for Sentence Correctness

Identifying the grammatically correct sentence requires understanding two key grammar points: pronoun case and subject-verb agreement, especially within relative clauses.

Pronoun Case After Linking Verbs

In sentences structured like "It is + Pronoun", the pronoun typically functions as a subject complement. Standard English grammar requires the subjective case pronoun (e.g., I, he, she, we, they) in this position, rather than the objective case (e.g., me, him, her, us, them).

• Correct usage: $It is I$.
• Incorrect usage: $It is me$.

Subject-Verb Agreement in Relative Clauses

The relative pronoun 'who' connects a dependent clause to its antecedent. The verb used with 'who' must agree in number and person with its antecedent. In the construction "It is [Pronoun] who...", the verb must match the antecedent pronoun.

Analysis of Options

1. Option 1: It is I who am responsible for this fiasco.
   • Pronoun Case: 'I' is the correct subjective case after 'is'.
   • Verb Agreement: 'who' refers back to 'I'. The verb must agree with 'I', making 'am' the correct verb. This sentence is grammatically sound.
2. Option 2: It is myself who is responsible for this fiasco.
   • Pronoun Case: 'Myself' is a reflexive pronoun and is not used as a subject complement in this context. It is incorrect.
3. Option 3: It is I who is responsible for this fiasco.
   • Verb Agreement: Although 'I' is correct, the verb 'is' does not agree with 'I'. The correct verb should be 'am'.
4. Option 4: It is I who are responsible for this fiasco.
   • Verb Agreement: Although 'I' is correct, the verb 'are' does not agree with 'I'. The correct verb should be 'am'.

Final Determination

Only Option 1 correctly applies both the rule for subjective pronoun case after 'is' and the rule for subject-verb agreement within the relative clause ('who' takes a verb that agrees with 'I').