过去完成进行时 Past Perfect Continuous

表示在过去某一时间或某一动作发生之前，一直在持续进行的动作。

• 结构公式：主语 + had been + doing
• 否定式：主语 + had not been (hadn't been) + doing
• 疑问式：Had + 主语 + been + doing...?
• I had been doing my homework for 2 hours before you opened the door. —— 在你开门之前，我已经做了两个小时的家庭作业了。
• The road was muddy because it had been raining heavily. —— 道路泥泞，因为之前一直在下大雨。
• He was tired because he had been working all night. —— 他很累，因为他之前熬夜工作了一整晚。

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