Please see this post which contains a lot more information on how and when to use the perfect tenses.
To briefly answer your question: As you say, the perfect tense is used to relate two actions or imply a time relationship between now and some action. "When I was younger" is an adverbial phrase, not a separate action. All it does is tell you when something happened, no different from saying "yesterday" or "last year" or "in prehistoric times". Just setting a time frame for an action isn't enough to justify using the perfect tenses -- there has to be a good reason to relate two actions. For example:
When I was younger, I had wanted to be a doctor.
Incorrect. Simple past is sufficient. "I wanted to be a doctor".
When I was younger, I had wanted to be a doctor -- until I saw the movie "Top Gun". Then I wanted to be a fighter pilot.
Correct. "Had wanted" implies it was true, but something happened to affect that desire.
With this information you should understand why "I had gone to the store" is incomplete. You need to have something else happen to justify using the past perfect tense:
I had gone to the store when I remembered I was late for an appointment.
I had just gone to bed when there was a loud noise outside.
I'd started the exam before I discovered I'd forgotten all my pencils at home.
As with most things in English there are subtleties and exceptions with the perfect tenses that you have to learn with practice. Hopefully this will be enough to get you started.
I think the sentence is wrong, it should either be "they had been playing football since 10 o'clock, meaning they started at ten and continued until a later time, or "they were playing football at ten" meaning at ten they were in the middle of the game. I think it is a mistake because the other sentences are all examples of the past perfect continuous.
Best Answer
When you see a sentence that has the auxiliary be + past participle, it means that the sentence is in the "passive voice". You might know the "auxiliary be" as "verb to be", which means the verb to be in any of its form, and was is one of them. You seem to already know what past participle is, which refers to the verb form such as written, sung and raised; and because most of the part participles are regular (having the -ed suffix), the "past participle" is usually called the "-ed form", or the "verb -ed".
When A does something to B, you have two ways to express it: in active, or in passive voice. Consider:
You mentioned the confusion of mistaking the passive voice with the past continuous (was,were + verb -ing) and the past perfect (had + past participle). I think it's likely that you are also confused by the present perfect (has,have + past participle). Here is how the confusion was discussed in Practical English Usage, under the entry 412.5:
I believe that it can address your confusion nicely.