Sometimes the time of the simple past action is implied. This means that you don't see it in the same sentence, but you understand it from the context (ie. the time has usually been given already).

Could you please correct your own sentence? I've underlined the errors.

"But here some example that cast a frustration over me, cause I don't know where did this come from."

Radoslav

I understand why you are asking these questions about the present perfect tense... it is difficult to understand and not exactly easy to explain, but I wil try.

"I have been in London once."

It is true that it happened once (and the one visit is completed), but there is still the continuing chance that it may happen again, so the present perfect (not 'past perfect') tense is appropriate.

So yes, the present perfect tense is appropriate.

"I was in London once."

This talks about the one time that you visited London... it is simply a statement of fact.

It does not suggest that you may go there again.

But if you use the present perfect, it suggests that you may go again in the future:

"I have been in London once (and I may go again someday)."

Again, I hope this helps...

Radoslav... Paige said it best... you graduated ONCE (and the graduation did not continue).

The following is not a criticism, simply a couple of suggestions:

Hi everyone. I face big difficulties deciding which tense to use, the Present Perfect or Simple Past.

Is there a methodology to help me differentiate between them?

When a specific time is used into in the predicate, I have no problems, I just use the simple past.

Here this is an example that induces a lot of frustration to in me,

Here is an example/This is an example  that frustrates me

because it seems to contradict one of the rules that define the simple past tense.

Hope this helps...

Thank you @Richard.
Contemplating on your explaination " you graduated ONCE (and the graduation did not continue)" new questions emerge. For example:

I have been in London once.

Is the ^^above ^^ sentence grammatically correct? If yes, then it(being in London) happened once and it does not continue. But it is still Past Perfect.

I am open for more criticism :)