The general rules of the Serbian plural depend on its gender, ending and syllable but like in other languages, there are exceptions to these rules

Masculine 

(adding i)

phone - phones

telefon - telefoni

computer - computers

kompjuter - kompjuteri

(adding - evi)

( When the word is one syllable, and ends with either ž,đ,š,ć,č,nj,lj, j,c add evi )

knife - knives

nož - noževi

mouse - mice

miš - miševi

(adding - ovi)

( When the word is one syllable, and NOT ending with ž,đ,š,ć,č,nj,lj, j,c add ovi )

city - cities

grad - gradovi

wolf - wolves

vuk - vukovi

There are exceptions for this one syllable rule like:

pas - psi

dan - dani

sat - sati

Collective Plurals

tree - a bunch of trees

drvo - drveće

a flower - a bunch of flowers

cvet - cveće

a leaf - a bunch of leaves

list - lišće

Feminine

(changing a to e)

house - houses

kuća - kuće

chair - chairs

stolica - stolice

Feminine (ending with consonant, add i )

word - words

reč - reči

thing - things

stvar - stvari

Neuter

(changing e/o to a)

egg - eggs

jaje - jaja

place - places

mesto - mesta

Plurals without Singular

Vrata - door

novine - newspaper

naočare - glasses

pantalone - pants

usta - mouth