Past Perfect Continuous Tense

1. Для выражения действия, которое происходило в прошлом до к.л. прошедшего момента, если нужно подчеркнуть длительность действия или показать, что оно захватило и упомянутый момент времени в прошлом: When he came to the school m 1950, Green had already been teaching there for 5 years. She had been waiting for an hour by five. By 1990 I had been working here for 5 years.
2. По согласованию времен вместо Past Continuous: He said, "I was working from 9 till 12" = He said he had been working from 9 till 12.