What is the difference between present perfect and past perfect? — Laxman, India
The present perfect tense says that an action was completed at a time before the present, and the results or consequences of the action are relevant now. The present perfect is formed using the present tense of the verb "to have" and the past participle of the main verb.
The past perfect tense says that an action was completed at a time before another action happened in the past. The past perfect is formed using the past tense of the verb "to have" and the past participle of the main verb.
Let's use a timeline to look at how the present perfect and the past perfect relate to one another. Point A will be the action of the kids eating dinner. Point B will be when the nanny goes home. Point C will be now, the present. We are always speaking from point C.
past ←----------A----------B----------C----------→ present
Present perfect: The kids have eaten dinner. [They ate dinner at point A, it is point C now. Note: For the present perfect, point B does not need to exist. A and C are the only points that matter.]
Past perfect: The kids had eaten dinner before the nanny went home. [The kids ate dinner at point A, the nanny went home at point B, it is point C now.]
The difference is that in the present perfect example, the result of A is that the kids do not need to eat now, at point C, but in the past perfect example, the result of A is that at point B, no one needed to prepare dinner for the kids.