Describe the past perfect continuous tense and give examples of its usage.

The past perfect continuous tense, also known as the past perfect progressive tense, is used to describe an ongoing action that started in the past and continued up until another point in the past. It is formed by using the auxiliary verb "had" in the past perfect tense, followed by "been" and the present participle form of the main verb (-ing form).

The past perfect continuous tense is typically used to emphasize the duration of an action or event that occurred before another past action. It helps to establish a clear sequence of events in the past.

Here are some examples of the past perfect continuous tense in usage:

1. She had been studying for three hours before she finally took a break.
In this example, the action of studying started in the past and continued for a specific duration of three hours before she decided to take a break.

2. They had been waiting at the airport for hours when their flight was finally announced.
Here, the action of waiting started in the past and continued for an extended period of time until their flight was announced.

3. By the time I arrived, they had been playing tennis for two hours.
This sentence indicates that the action of playing tennis started before the speaker's arrival and continued for a duration of two hours.

4. The children had been playing outside all day, so they were exhausted by evening.
In this example, the action of playing started in the past and continued throughout the day until evening, resulting in the children feeling exhausted.

5. He had been working on the project for weeks before he realized he had made a mistake.
This sentence suggests that the action of working on the project began in the past and continued for several weeks until the realization of a mistake occurred.

In summary, the past perfect continuous tense is used to describe ongoing actions or events that started in the past and continued up until another point in the past. It helps to establish a clear sequence of events and emphasizes the duration of the action or event being described.