Past perfect continuous of slip

The past perfect continuous tense (sometimes referred to as the past perfect progressive tense) is specific type of tense in English. It is easy to learn but harder to use. As we already know, the past perfect continuous also describes events that „passed in the past“ – just like the past perfect tense.

The only difference is that the past perfect continuous describes events that started, lasted but also ended at some point in the past.
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