A reader wanted to know how to use the word had in relation to verb tense, so I’m going to try to give a simple, short answer to her question. I think what the reader was asking about was the past perfect tense.
The past perfect is used when two events happened in the past, with one past action having occurred even before the other past action. To form the past perfect, use had and the past participle of a verb in one part of the sentence. Often, the regular past tense is used in the other part of the sentence.
Sally had agreed to wait in the pumpkin patch with Linus before she realized that there was no such thing as the Great Pumpkin. Both events happened in the past—agreeing to wait in the pumpkin patch and realizing that there was no Great Pumpkin—but the agreeing happened even before the realizing, so we have to use the past perfect tense for the agreeing part and the regular past tense for the realizing part.
We might be tempted to say Sally agreed to wait in the pumpkin patch with Linus before she realized that there was no such thing as the Great Pumpkin, but we’d be wrong. Just using the regular past tense for both parts of the sentence doesn’t work because one event happened before the other event.
Here are some other examples:
The telephone rang after we had left the house. (Both the phone ringing and the leaving occurred in the past, but one occurred even more in the past than the other.)
He had been to Paris, so I asked him whether I could get by without learning French. (He went to Paris in the past—he’s now back—and I asked him a question in the past.)
By the time Lucy woke up, Linus had fallen asleep in the pumpkin patch.