Ответ:

1 I couldn't remember where I had left my keys.

2 Last Saturday I ate at a Japanese restaurant. I had never eaten Japanese food before that.

3 I found a 10-pound note that I had lost last year.

4 Last weekend I met a girl who had been born in the same hospital as me.

5 I didn't play volleyball because I had hurt my hand.

6 The pavement was wet because it had rained.

Объяснение:

Если в предложении есть указание на то, когда совершено действие, то используется Past Simple.

Если в предложении есть два действия, которые произошли в прошлом, то более раннее действие выражается при помощи Past Perfect.