The past perfect continuous tense is used to describe actions that started in the past, continued for some time, and were still ongoing up until another point in the past. It emphasizes the duration of an action up to a specific past moment.

Structure
The structure of the past perfect continuous tense is:

Subject + had been + verb(-ing)

Usage of past perfect continuous tense - 

1. To show an action that was ongoing until a specific point in the past  
   - Example: They had been studying for hours before the test started.
     - (This emphasizes the length of time they were studying before the test.)

2. To describe the cause of something in the past  
   - Example: She was tired because she had been working all day.
     - (This implies that the continuous work earlier in the day caused her to feel tired.)

Examples of past perfect continuous tense - 
- I had been waiting for over an hour when he finally arrived.
- They had been living in that house for ten years before they moved.
- We had been walking for miles when it started to rain.

Time Expressions Often Used -
Common time expressions with the past perfect continuous tense include:
- for + a period: for two hours, for several years
- before, until: before he arrived, until she moved
- since + a specific point in the past: since 2010, since morning

Key Points
- Focuses on the duration of an action that continued up to a specific past time.
- Requires had been followed by a verb in the -ing form (present participle).
  
The past perfect continuous tense is especially useful when you want to emphasize the duration or ongoing nature of an action that led up to a past event.