The past perfect: had done

had + past participle

The story is at one moment in the past. The past perfect looks back from that moment to something that happened before it.

Short forms: I'd / you'd / she'd / they'd + past participle (I'd booked = I had booked) · hadn't = had not

• by the time + past simple, then the past perfect: By the time we found a parking spot, the game had started.
• already, just, never go between had and the participle: had just left, had never driven.
• when + past simple: When I opened my bag, my wallet had disappeared.

It changes the story

• When I looked out the window, the bird flew away. (I saw it go: first I looked, then it flew.)
• When I looked out the window, the bird had flown away. (It was already gone when I looked.)

With before and after, the order is already clear, so the past simple is fine too: I locked the door before I left.

Watch out

• had + past participle, not the past simple: had went → had gone; had took → had taken.
• It needs a past moment to look back from. About your life up to now, use the present perfect: I have never seen a tornado. (not I had never seen)
• had alone is the past of have: We had lunch at noon. Its past perfect is had had: By noon, she had had three cups of coffee. And 'd can be had or would: she'd left (= had left), she'd like tea (= would like).