Yeap, this is very hard to be explained by a native speaker since we use these forms naturally and don't think if we use "perfect aspect" or "secondary imperfect aspect" of the verbs. I saw Psi-Lord (also a member of Unilang who has disappeared without trace) has asked the same question in another forum: http://forum.wordreference.com/showthread.php?t=1195305
. I hope it's useful.
I confirm that the difference in the meaning is very small and which form is correct is more or less a grammatical question than lexical.
I'll give you an example (but I'm not sure if this would help a lot)
(imp.) Докато те няма, ще мета двора
(всеки ден). - While you're away I'll sweep the yard
(every day). (That means I'll sweep the yard but it's not sure that I'll sweep it the whole every day).
(perf.) Докато те няма, ще измета двора
. - While you're away I'll sweep the (whole) yard
(but only once
(sec. imp.) Докато те няма, ще измитам двора
(всеки ден). - While you're away I'll sweep (the whole) yard
More clear now?