There is a combination of certain consonants and vowels that leads to changes:

(a)

k + i -ci	(momak, momci)
g + i -zi	(kovčeg, kovcezi)
h + i -si	(juha, u jusi)

These changes occur in masculine plural and feminine dative/prepositional of nouns; and the imperative (pomoći - pomog-ti - pomozi, pomozite!).  NB this rule does not apply to adjectives.  Exceptions include proper names and feminine nouns with stem ending in -tk (tetka), -čk (mačka) and -zg (tezga)

(b)

These changes occur in the vocative singular of masculine nouns; and in the present tense of verbs with infinitive in -ći derived from k + t, g + t and h + t:

pecći (pek-ti)	pečem	(to roast)
strići (strig-ti)	strižem	(to shear)
vrći (vrh-ti)	vršem	(to thresh)

(c) consonant + j.  These changes occur mostly in the comparison of adjectives.

p	+	j plj	skup	-	skuplji	expensive
b	+	j blj	riba	-	riblji	fish
v	+	j vlj	krv	-	krvlju	blood
m	+	j mlj	razum	-	razumljiv	reason - comprehensible
t	+	j ć	smrt	-	smrću	death
d	+	j đ	mlad	-	mlađi	young
s	+	j š	visok	-	viši	tall
z	+	j ž	brz	-	brži	fast
k	+	j č	jak	-	jači	strong
h	+	j š	tih	-	tiši	quiet
g	+	j ž	drag	-	draži	dear
st	+	j šć	cest	-	češći	frequent
zd	+	j žđ	grozd	-	grožđe	grapes

Assimilation.  As the spelling of Croatian and Serbian is phonetic, any consonant assimilations that occur are recorded in writing.

(a) alternation of voiced/unvoiced consonants: the following consonants are 'paired':

unvoiced:	č	ć	(f)	k	p	s	š	t
voiced:	dž	đ	(v)	g	b	z	Ě	d

If one of these consonants is placed immediately next to one of the others it may be replaced by its pair so that they are both eithr voiced or unvoiced, according to the last consonant in the group:

sladak	slatko	sweet
težak	teško	difficult
Englez	engleski

(b) alternation of s and z with š and ž

When s, z are placed next to a palatal or palatalised consonant, they change to their palatalised equivalent š and ž:

misliti	to think	mišljenje	opinion
paziti	to take care	pažnja	attention care