The past perfect and past perfect continuous are often used together to talk about an event in the past, although the past perfect continuous can, of course, be used on its own.
The past perfect continuous form is:
S (I/you/we etc.)+ had + past participle + verb-ing
Here are a couple of its uses:
1). The past perfect continuous talks about the actions that occurred in a length of time in the past.
Check the following examples:
Reginald had been watching T.V for 5 hours; what had you been doing before you arrived at the party?; we had been seeing each other for 5 months
2). And whereas the past perfect describes how many of something there was, the past perfect continuous tends to talk about a length of time. The tenses, therefore, tend to complement each other.
Check out the below example:
The main action the lion was doing (hunting) is described using the past perfect continuous. We could be specific and say the lion had been hunting from 2 PM to 5 PM, but we tend to use the past perfect to get into the details of the past perfect continuous action.
Check this following example also:
Again, the past perfect continuous describes the overall action occurring in a time period in the past (watching a football game) and the past perfect adds on details of actions that happened during that time (we hadn't conceded, Kane had scored a hattrick).
It's, therefore, useful to use the two tenses together to describe events in the past.