Choose the correct verb form: 'If he ______ harder, he would have passed the exam.'
答案解析
**Core Concept:** Grammar - Third conditional structure.
**Analysis:** The third conditional requires 'had + past participle' (D) in the 'if' clause for hypothetical past actions.
**Wrong Options:**
- A (simple past) and B (present) mismatch the hypothetical context.
- C incorrectly uses 'would' in the conditional clause.
**Key Clue:** 'Would have passed' indicates a past unreal condition.
正确答案：D