Past Perfect Tense

Past Perfect Tense is used for the situations which happened in the past. These situations took place before other activities which also happened in the past. It is not neccessary to describe these both activities in one sentence. If it is absolutely clear from the context that we deal with two events and we want to stress that one of them took place first, then Past Perfect Tense should be used. The sentences below show the context for the tense in questions.

When we arrived they had gone home.
After she had passed all the exams she got her diploma.
She explained what she had meant.
I had never been to the UK.