Learn English – Using Past Perfect in English conversation

No. Apparently not. The relevance of Past perfect lies in the fact that it differentiates between the time frame of two events, when both of them happened in the past.

Example:

Before she left for her college, she had finished her lunch.

You can use After too. In the given sentence in your question,

Mario had already arrived home when his mother walked in.

it is understandable that Mario had reached home before his mother walked in.

BUT a simple,

Mario had arrived home.

is meaningless and unnecessary as you are not referencing before what event had he arrive home. You can use simple past here. Of course, if in a conversation it is relevant that you did it before some action you can use simple perfect like you cited. Like:

2nd friend: Where do you work now?

1st friend: I work in ABC company now-a-days.

2nd friend: Oh! I see. I had worked there. (It means he had worked there before he left that job)

So put it simply,

Past perfect is not used unless and until we are comparing the time frames between two events. You can read more on past perfect in English Page: Past Perfect Tense.

First past perfect and pluperfect refer to the same tense. They are synonyms. Look up pluperfect in any dictionary to verify this. Here at onelook dictionary are more than two dozen dictionaries to get you started.

Second, please see this answer to When is the past perfect exactly needed? It does a terrific job of stating when the past perfect (or pluperfect--they mean the same thing) is necessary.

In general your sentence 1B is grammatical, but it represents rather an incomplete thought. And it is not necessary to use the past perfect/pluperfect tense here. The present perfect or simple past would both be better choices, because they would not represent incomplete thoughts.

Your 1C adds more information to the response but still either the present perfect or simple past would be more common. You don't need the past perfect/pluperfect there because it is clear from logic which past action happened first, namely that you called him first.

In 2B, there is no reason for using the past perfect/pluperfect tense. The simple past is fine. You mention only one past action. And in fact both uses of the verb order refer to that same past action. It would seem strange to think that one occurrence of the same past action can come before another occurrence of the same!

The first sentence of 2C is grammatical But by itself gives an incomplete thought. There is only one past action mentioned, and it occurs two days back. Thus the simple past is better here because you are talking about one action completed in the past. Having us wait until the second sentence to hear the most recent past action (Yesterday when I opened the box) makes it awkward.

The second sentence of 2C is the only sentence in which the past tense/pluperfect tense is used in a way that is intended. You have two actions and you place one of them before the first one. This is pleasant sounding and appropriate.

For example sentences and a much better explanation, see the answer I link to above. And thanks for asking such a good question.