From a future exchange student

[tccam]

For past tense biti, it's

bio, bila, bilo?


How do you choose the perfective or imperfective for the infinitive?

[Zorba]

There are various past tenses in BCS: perfect, aorist, imperfect, and pluperfect. The most common is the perfect tense (it fulfills the functions of English Past Simple and Present Perfect). This is formed by using the conjugated form of the clitic of the verb biti and the 'L' participle in the correct number and gender. To form the L participle, drop the suffix -ti, -sti or -ći and add these suffixes:

-o = M Sing --- -li = M Plural
-la = F Sing --- -le = F Plural
-lo = N Sing --- -la = N Singular

So "I ate" would be "(ja) sam pojeo" or "(ja) sam pojela" in feminine. "we thought" would be "(mi) smo mislili". "you were" would be "(ti) si bio" or "(ti) si bila" in feminine. And so on.

Aspect is difficult to explain in a single post. Try reading this article to start with and come back if you have specific questions:

http://www.phrasebase.com/archive/serbi ... enses.html

[kibo]

To form the L participle, drop the suffix -ti, -sti or -ći and add these suffixes:

-o = M Sing --- -li = M Plural
-la = F Sing --- -le = F Plural
-lo = N Sing --- -la = N Singular

Actually, it's a bit more complicated than that when it comes to verbs in -sti and -ći. (yes, those verbs are always special ) In order to know how make the participle of these verbs you should know their present tense form, specifically the 3rd person plural.

peći - ja pečem - oni peku - participle: pekao, pekla, peklo, pekli, pekle, pekla
moći - ja mogu (ti možeš) - oni mogu - participle: mogao, mogla, ...
gristi - ja grizem - oni grizu - p.: grizao, grizla, ...

As you can see you have -ao in the masculine singular form and not -o.

Now the irregularities:
When the verbs in -sti end in -t(n)em or -d(n)em in the present tense, the participle is formed by dropping the -t(n)- and -d(n)-, just like in the example you gave
jesti - ja jedem - oni jedu - p.:jeo, jela, ...
krasti - ja kradem - oni kradu - p.: krao, krala, ...
pasti - ja padnem - oni padnu - p.: pao, pala, ....

The verbs in -ći which end in -nem in the present tense drop the -n-
leći - ja legnem - oni legnu - p.: legao, legla, ...

ići, otići and all the verbs in -ći which end in -đem in the present tense get -šao, -šla, -šlo, -šli, -šle, -šla.

ići - p.: išao, išla, ....
naći - ja nađem - p.: našao, našla, ...
doći - ja dođem - p.: došao, došla, ...

NB: And this is the "recipe" for the Ekavian variant, the verbs in -ěti complicate things even more in the Ijekavian one.