EXERCISE 27–10Verb tense and mood
Example:

After the show had begun, the crew noticed that one crucial prop was missing on the stage.

After the show began, the crew noticed that one crucial prop was missing on the stage.

In the first sentence, the past perfect tense (had begun) is used correctly for a past action that occurred before the past action in the independent clause (noticed).

For more help, see section 27f.