过去完成时是一个相当容易理解和使用的一个时态。这个时态谈论的是“过去的事”（ "past in the past"）
The auxilary verb (have) is conjugated in the Past Simple: had
The main verb is invariable in past participle form: -ed (or irregular)
When we use the Past Perfect in speaking, we often contract the subject and the auxiliary verb. We also sometimes do this in informal writing:
I'd eaten already.
They'd gone home.在否定句中，我们缩写助动词和“not”：
I hadn't finished my meal.
Anthony hadn't had a day off for months.
The train left at 9am. We arrived at 9: 15am. When we arrived, the train had left.
I wasn't hungry. I had just eaten.
They were hungry. They had not eaten for five hours.
I didn't know who he was. I had never seen him before.
"Mary wasn't at home when I arrived." / "Really? Where had she gone?"
You can sometimes think of the Past Perfect tense like the Present Perfect tense, but instead of the time being now the time is before.
"You are too late. The train has left."
"We were too late. The train had left."
我们经常在转述的言语中使用过去完成时，跟在动词后面，比如：said, told, asked, thought, wondered
He told us that the train had left.
I thought I had met her before, but I was wrong.
He explained that he had closed the window because of the rain.
I wondered if I had been there before.
I asked them why they had not finished.