O Past Perfect (had + past participle) descreve uma ação que aconteceu antes de outra ação no passado. É o 'mais-que-perfeito' do inglês.
When I arrived, she had already left. (ela saiu antes de eu chegar)
He hadn't eaten anything, so he was very hungry.
By 9pm, they had finished the meeting.
A mesma forma para todas as pessoas: had + particípio passado.
I/You/He/She/We/They had finished.
Negativa: She hadn't seen him before.
Pergunta: Had they met before? — Yes, they had.
Use o Past Perfect para a ação mais antiga. Use o Past Simple para a mais recente.
After he had finished dinner, he went to bed.
She called me after the meeting had ended.
By the time I arrived, they had gone home.
Confusões comuns para falantes de português: