The -l form is not the past participle. I don't know the term for this but it's different from what the past participle can exactly mean. There is a much more rich palette of participles in West Slavonic languages: adjectival (present: a working man, past: a written letter) and adverbial (present: he talked to me, writing a letter, past: he talked to me, having writtena letter). I don't know how it looks in Bulgarian but it is probably handled much more simply.Adder, as far as I know all slavic languages (well, I know nothing about Kashubian, Sorbian and other exotic lingos) except BG use past participle + the verb 'to be' to form a past tense.
What I wrote was the construction of the past tense is formed this way:
the present form of 'to be' + the -l form
Russian doesn't attach 'to be' in any way so the use of personal pronouns is necessary.
It's pisałem. '-em' is a shortcut of the verb 'to be' attached to the -l form.'pisał + something'
To sum up, this is the formation of the tense that is known as Perfectum. I call this Perfectum because of the way it is formed, not because of the action it involves as it can be both perfective and imperfective which is only aspect.
The next thing is I've read Bulgarian, Serbo-Croatian (and Macedonian) still use Simple Past Tenses from Old Slavonic which are Imperfectum and Aoristus. And the usage of these is what I would be interested in.