Past perfect or Present perfect tenses

Anybody can help me to understand the difference between present perfect and past perfect? Although I know that the action in the past perfect happened earlier in time than the other action, which is in the simple past. But I often see that past perfect is used on its own, where there isn't any other action. In this case why can't we use present perfect?

Strictly speaking, the past perfect is only used when referring to something that happened before something else in the past. So whenever you see the past perfect being used on its own, it's wrong; you have to use the simple past instead (but a lot of people mistakenly do it, even fluent speakers).

e.g. He had studied enough, so he failed the exam.

He did not study enough.

The present perfect is used for something that either has just happened or still is affecting the present. Usually it's used with words like "just", "already", "yet", etc.

Compare:
I have just eaten the cake.
I ate the cake (a long time ago).

I have lived in Melbourne for 3 years (and I'm still living there).
I lived in Melbourne for 3 years (and I no longer live there).