There are four participles in Croatian and Serbian, of which two are verbal adjectives and two verbal adverbs.

We have met the active past participle (bio, imao, etc), and know that this is one of the adjectival forms.  We shall come to the other adjective form, the passive past participle, later - it is not in very common use.

Verbal adverbs or gerunds.  There are two, one denoting simultaneous action and the other past action: (i) 'seeing' (ii) 'having seen'

Present gerund

This is formed by adding the suffix -ći to the third person plural of the present tense of the imperfective:

(oni) govore	govoreći	speaking
(oni) pišu	pišući	writing
(oni) imaju	imajući	having

The gerund is infrequently used, even in the written language, except in certain situations.

The present gerund of biti is based on the perfective third person plural budu = budući.  This is used frequently as a conjunction with da:

Budući da smo u Hrvatskoj . . .  -  Since we are in Croatia . . .

Budući da je lijepo vrijeme . . .  -  SInce it is nice weather . . .

The gerund is strictly speaking, indeclineable (like all adverbs).  But certain gerunds are becoming increasingly used as adjectives:

budući	Da te upoznam sa svojim budućim mužem! Let me introduce you to my future husband!
idući	Vratit će se sigurno iduće godine ako mogu They will certainly come back next year if they can
slijedeći	Slijedećeg dana smo se rano probudili We woke up early the following day
tekući (teći 'to flow')	Koliko imaš na tekućem račnu? How much have you got in your current account?

Past gerund

This is formed by adding the suffix -v, -vši (after a vowel) or -avši (after a consonant) to the infinitive stem:

usta-ti	ustavši	rek-ti (reći)	rekavši
vidje-ti	vidjevši	mog-ti (moći)	mogavši
sjeti-ti se	sjetivši se

Again, the past gerund is rarely used: it is more common to find a whole clause where such a participle is used in English.

      Having started the book, I'd like to finish it
      Kad sam počeo čitati knjigu, htio bih je završiti
or  Budući da sam počeo . . .
or  Počeo sam, pa bih htio . . .

As in the case of the present gerund, we find that the past gerund of biti, bivši, is in common use as an adjective:

Da te upoznam sa svojom bivšom suprugom!  -  Let me introduce you to my former wife!
Ta kuća pripada bivšem predsjedniku  -  That house belongs to the former president

NB:  The common English construction with the active present participle ending in '-ing' can cause problems in translating eg, 'I heard the birds singing'.  Such sentences should be rendered by kako + present (imperfective) tense:  Čuo sam kako ptice pjevaju.

Passive past participle, verbal nouns

Passive past participle - this is the other verbal adjective.  Passive participle are formed from the infinitive stem of either aspect.  Most verbs with infinitive ending in -ati have passive participle in -an:

čitati                     čitan, čitana, čitano
pozvati                 pozvan, pozvana, pozvano

Most verbs with infinitive ending in -ti or -eti/jeti have -jen

vidjeti                    viđen, viđena, viđeno
učiniti                   učinjen, učinjena, učinjeno

Verbs with infinitive ending in -nuti have -nut:

prefinuti                prekinut

Verbs with infinitive ending in -eti and present -mem or -nem have -et:

zauzeti (zauzmem)       zauzet

prokleti (prokunem)     proklet

These are the most common regular patterns.  Examples:

Je li ovaj kruh dobro ispečen?  -  Is this bread well baked?

Stol je prostrt, kruh je narezan, vio je ohlađeno, riža je kuhana.  Dođite ručati!
The table is laid, the bread is cut, the wine chilled, the rice boiled.  Come and have lunch!

Verbal nouns.  Many of these passive participles may be made into nouns by the addition of the suffix -je:

osloboditi to liberate	oslobođen liberated	oslobođenje
čitati	čitan	čitanje	reading
promatrati	promatran	promatranje	observation, watching
učiti	učen	učenje	learning