If you do not include a duration such as "for five minutes," "for two weeks" or "since Friday," many English speakers choose to use the past continuous rather than the past perfect continuous. Be careful because this can change the meaning of the sentence.
Past continuous
emphasizes an interrupted action
He was tired because he was exercising so hard.
(This sentence explains that he was tired because he was exercising at that exact moment.)
Past perfect continuous
emphasizes a duration of time before something in the past
He was tired because he had been exercising so hard.
(This sentence stresses that he was tired because he had been exercising over a period of time. It is possible that he was still exercising at that moment OR that he had just finished.)
When the time period is given («for hours», «since last month») always choose past perfect continuous.
Past continuous
an interrupted action
We were playing football when it started to rain.
Past perfect continuous
emphasizes a duration of time
We had been playing football for two hours when it started to rain.
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