I cannot get the gist of your question. By presuming that both languages are slavic, there must be a lot in common between them. The differences are that Bulgarian has (has developped) much more past tenses than Russian. You need to ask a more specific question.
In fact, був is the past participle like бачив, the auxiliary есмь is still missing.Modern Ukrainian - Я був бачив or Я бачив був
In fact, був is the past participle like бачив, the auxiliary есмь is still missing.
With the auxiliary есмь it could be something like Я есмь був бачив. In Czech it is Já jsem byl (u)viděl (bookish) as the Czech past tenses preserve the auxiliary verb to be in the 1st and 2nd persons.
The Bulgarian бях in your example is the imperfect.
I don't know who said it.Who said an auxialiary verb CAN'T be used in its past participle form and still play the auxiliary part?
Interesting. It resembles Polish. So the hypothetical plusquamperfect would be: Я бувемь бачив.Ходивемь (ходив єсмь)
Who said an auxialiary verb CAN'T be used in its past participle form and still play the auxiliary part? Був is, indeed, the p.p. of бути. Nevertheless, it is used to form the Ukrainian plusquamperfect as an auxiliary verb, at that it doesn't have any sematic meaning (expresses no action) here.
Speakers of some Western Ukrainian dialects (Hutsuls, Rusyns etc.) use the present tense of the auxiliary verb бути to form the past (or, more precisely, the present perfect). The auxiliary is postpositioned.
Ходивемь (ходив єсмь)
Ходивсі (ходив єсі)
Ходилисьмо (ходили єсмо)
Ходилисте (ходили єсте)
The бях (беше, беше, бяхме, бяхте, бяха) in my example is the so-called GENERAL PAST that's used to form the PLUSQUAMPERFECT in the Bulgarian language.
The PAST IMPERFECT forms of the Bulgarian verb съм are:
бъдех, бъдеше, бъдеше, бъедехме, бъдехте, бъдеха
The AORIST forms of the Bulgarian verb съм are:
бидох, биде, биде, бидохме, бидохте, бидоха
The ARCHAIC AORIST forms of the Bulgarian verb съм are:
бих, би, би, бихме, бихте, биха.
Of all of the above, only the бях беше, беше, бяхме, бяхте, бяха forms are used in the Bulgarian plusquamperfect.