I had already eaten dinner.

They had already left.

I had already finished my homework.

She had already been to that place.

By the time I went outside, the bus had already left.

When I got to school, I realized I had left my backpack at home.

No sooner had we left the house than it began to rain.

I had been looking for it for days before I found it.

The telephone had been ringing for three minutes before it was answered.

He had finished reading the novel by nine o'clock last night.

这些例句展示了过去完成时在不同语境下的应用，包括表示在过去某一时间点之前已经完成的动作或状态。