The past perfect tense of the verb ‘fell’ is ‘had fallen.’
To understand this, it’s important to break down the components. The verb ‘to fall’ is an irregular verb, and its past simple form is ‘fell.’ The past perfect tense is used to describe an action that was completed before another action in the past. It is formed by using ‘had’ plus the past participle of the verb.
In this case, the past participle of ‘fall’ is ‘fallen.’ Therefore, when you want to indicate that something had already happened before another past action, you would use ‘had fallen.’ For example: ‘By the time I arrived, the leaves had fallen.’