He_______________us but he was short of money at the time.
Ta dùng lời suy đoán cho tình huống ở quá khứ (deduction in the past): must/may/could/would have + past participle. "He could have helped us but he was short of money at the time." (Lẽ ra lúc đó anh ấy có thể giúp chúng ta nhưng anh ta lại kẹt tiền.)