Ako prijateljici slučajno kažem nešto loše, poslije joj se ispričam.

Because Croatian says reći nešto nekome — literally to say something to someone.

So prijateljici is the dative singular of prijateljica and means to a female friend / to my friend depending on context.

• prijateljica = female friend
• prijateljici = to a female friend

A native English speaker often expects a direct-object form here, but Croatian treats the person who receives the words as an indirect object.

Because Croatian very naturally repeats the indirect object with a short pronoun.

Here both refer to the same person:

• prijateljici = to the female friend
• joj = to her

This is very common and natural in Croatian, especially when the second clause continues talking about the same person. English usually would not repeat it, but Croatian often does.

So the structure is basically:

• If I say something bad to my friend, afterward I apologize to her.

Yes, grammatically it is feminine, so it refers to a female person.

The base noun is prijateljica, which means female friend. In some contexts it can also mean girlfriend, but in a sentence like this it will usually just be understood as female friend unless context suggests otherwise.

If it were a male friend, you would expect prijatelju.

Because Croatian usually leaves subject pronouns out when they are already clear from the verb form.

Both verbs show first person singular:

• kažem = I say
• ispričam se = I apologize

So ja is not necessary. You could add ja for emphasis or contrast, but in a neutral sentence it is normally omitted.

Because Croatian commonly uses the present tense after ako for a general condition or a possible future situation.

So Ako ... kažem ... is normal and natural.

This is actually similar to English, which also often uses the present after if:

• If I say something bad...

not usually

• If I will say something bad...

In Croatian, ako + present is the normal pattern here.

Because reći is irregular in the present tense.

Its present forms are built from kaž-:

• kažem = I say
• kažeš = you say
• kaže = he/she says

So this is just something you have to learn as part of the verb’s conjugation. It is one of the very common Croatian irregular verbs.

Here slučajno means accidentally or by accident.

So slučajno kažem nešto loše means:

• I accidentally say something bad
• I say something bad without meaning to

It is an adverb, and Croatian is fairly flexible about where it goes, but this position is very natural.

Because nešto often takes a neuter singular adjective, and loše is the neuter singular form of loš.

So:

• nešto loše = something bad

Even though English does not show this kind of agreement, Croatian does. The adjective matches the grammatical pattern used with nešto.

That is why you do not get a form like lošu here.

Ispričati se means to apologize.

The se is part of the verb expression, so you should learn it as a whole:

• ispričati se = to apologize

Without se, ispričati usually means something like to tell, to narrate, or to tell completely.

So:

• ispričam = I tell / narrate
• ispričam se = I apologize

This is a very important difference.

Because joj and se are short unstressed words, called clitics, and Croatian places them in a special position.

Two important things are happening:

• they come near the beginning of the clause
• their internal order matters

So joj se is correct, not se joj.

Also, after the first stressed word poslije, the clitics naturally appear:

• poslije joj se ispričam

This kind of word order often feels unusual to English speakers, but it is very normal in Croatian.

Here poslije means afterward, after that, or later on.

In this sentence it connects the two actions:

1. I say something bad
2. afterward, I apologize

So poslije is not just random time in the future; it means after the first action.

Other words can sometimes be used in similar contexts, such as kasnije or nakon toga, but poslije fits very naturally here.

This is an aspect question.

• ispričam se is more perfective: it presents the apology as a complete action
• ispričavam se is more imperfective: it can sound more ongoing, repeated, or simply more neutral depending on context

In this sentence, ispričam se works well because it describes what I do as a complete response after I say something bad.

A learner should mainly remember this:

• ispričam se = I apologize / I do apologize
• ispričavam se = I am apologizing / I apologize

Both can appear in the right context, but they are not exactly the same in feel.

Yes, Croatian word order is flexible, but not completely free.

For example, you could also see things like:

• Ako slučajno prijateljici kažem nešto loše, poslije joj se ispričam.
• Ako joj slučajno kažem nešto loše, poslije joj se ispričam.

The exact word order changes emphasis and flow, but the clitics still have to follow Croatian rules. So the sentence can move around more than an English sentence, but not every order will sound equally natural.