Use had plus the past participle when one past action was already finished before another past action. Simple past is enough when the order is obvious or there is only one past time. She had studied before she sat for the test. Tense as a set is in verb tense.
Two pasts, one earlier
If the sentence already marks order with before or after, simple past on both verbs is often fine. The ACT wants had when the earlier action would otherwise look simultaneous. By the time the proctor spoke, he had already opened the booklet.
Grammar overall is in ACT grammar rules. Subject-verb still has to match, in subject-verb agreement. Lie versus lay past forms are a different trap, in lie versus lay.
The bottom line
Earlier of two past actions: had plus the participle. One past time: simple past.
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