Listen to the sentences and try and repeat them a case of its pronunciation practice. The difference in meaning ar well, the first one, when the bomb exploded, everyone left the building,. means the bomb exploded first, then people left the building. Means that there was no one in the building when the bomb explodes because they had already left. That's pretty straightforward, isn't it? Well, what i'd like to do now is just give you some sentences with a past perfect er in them - these will all be different. So try and listen to the difference between the clear sentence and the fluent sentence... Then try and notice exactly what the pronunciation pattern is, and then try and copy it