On the laboratory bench was a glass container from which came a tiny soft light. It was what they had been working so hard to find: pure radium.
上句中的 had been working 是什么时态呢？
上句中的 had been working 叫做过去完成进行时。它表示“从过去某一时间之前开始并一直延续到这个时间的动作”。它与过去完成时的相同之处：动作都是发生在某个过去时间之前。不同之处：过去完成进行时强调动作一直延续到某个作为前提的过去时间，而过去完成时强调动作在这个时间之前业已完成，即前者表“延续性”，后者表“完成性”：
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He was tired. He had been working all day. 他累了。他工作了一整天。(was tired 为过去时间的坐标)
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I had been looking for it for days before I found it. 我找了很多天才找到它。(found 为过去时间的坐标)
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He had been writing till two o'clock. 他一直写到两点钟。（“写”的动作是在两点之前开始的，到两点钟便告结束）
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How long had it been raining before you shut the window? 在你关窗之前雨下了多久？
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For a long time, I had been longing for a piano of my own to practise on. 我很长时间一直渴望自己能有架钢琴来练习。
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When she arrived I had been waiting for three hours. 她到达时我已经等了三个小时了。
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I realized that I'd been overworking, so I decided to take a couple of days' holiday. 我认识到自己工作过度，所以就决定度几天假。
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It had been raining heavily for two days and the ground was very wet. 大雨连续下了两天，(所以)地上非常湿。
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Robert Langdon awoke with a start. He had been dreaming. (Dan Brown, The Da Vinci Code) 罗伯特突然惊醒过来，他方才一直在做梦。（《达·芬奇密码》）
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For days he had been trying to sort through a barrage of information, but now Langdon found himself fixed on something he'd not considered before. (Dan Brown, The Da Vinci Code) 几天来他一直试图从林林总总的信息里理出个头绪来，然而现在，兰登发现他一心专注于他以前未曾考虑过的东西。(《达·芬奇密码》)