Core concept: Grammar - subjunctive mood in hypothetical conditions. Key analysis: 1. Subjunctive requires 'had + past participle' in if-clause for past hypotheticals 2. Correct answer (B) follows 'had known...would have approved' structure 3. Error analysis: - A/C use present tense for factual conditions - D mixes past tense with future consequence 4. Common mistake: Confusing real vs. hypothetical conditionals