There is no a distinction between these two phrases. The only way you can translate it in Russian is "Я съел яблоко". I ate an apple I have eaten an apple I had eaten an apple = all have the same translation

Tense is not the same as Aspect. You can't compare it with English Tense. Let me just give you some food for thought and give you examples: - Would you like an apple? ------Хочешь яблоко? - No, thanks! I HAVE already EATEN an apple today.(Present Perfect Tense)--------- Нет, спасибо, я уже сегодня ЕЛ (not съел) яблоко.( Imperfective Aspect) Now, an example with Past Perfect: I suddenly realized (Past Simple Tense) I hadn't eaten an apple today(Past Perfect Tense)------ Я вдруг поняла(Perfective Aspect), что не ела сегодня яблоко. (imperfective aspect) So, this idea of transferring English structure to Russian Aspect is completely wrong.

Verbs in Russian consist of the categories of TENSE and ASPECT. If you are asking if there is a distinction between perfect and pluperfect TENSE in Russian, then the answer in no, there is not. Russian has only one past tense and you use it to translate all English past tenses. However, the choice of perfective/imperfective ASPECT of the past tense in translation of English perfect tenses depends on the context.

The difference is contextual only, the Russian translation is the same for both, only the context is different. I have eaten the apple. = Я съел яблоко (до сих пор)/Я уже съел яблоко I had eaten the apple. = Я съел яблоко (до того как) - это предложение на английском не может существовать отдельно от контекста, который оправдает использование прошедшего перфекта. In Russian past indefinite, present perfect and past perfect will have one translation for all the 3.

There are no distinction in Russian. It can be "ел/съел" in both cases.