Answer:
After changing the sentence into the past perfect form, we have:
They had watched the movie (yesterday).
Explanation:
The past perfect tense is used to express actions that took place in the past before another action that also took place in the past. Take a look at the timeline below:
-----------------I -----------------I -----------------I-----------------I
past perf. s. past s. present future
The structure of the past perfect is: subject + had + verb (past participle). It is commonly used in sentences that also use the simple past, precisely because the past perfect expresses the action that took place first. For example:
- I had already called you twice when you finally showed up.
In the sentence we are changing here, I placed the adverb of time "yesterday" inside parentheses because it is usually employed with the simple past. However, depending on the context, it is possible for it to be used with the past perfect:
They had watched the movie (yesterday).
But it sounds better if we change "yesterday":
They had watched the movie the day before.
Both sentences would be correct.
