Moja klijentica kasni na sastanak, ali je javila da dolazi.

Because moja has to agree with klijentica in gender, number, and case.

• klijentica is feminine
• it is singular
• here it is in the nominative case, because it is the subject

So the possessive adjective is moja = my for a feminine singular noun.

Klijentica means female client. It is the feminine form of klijent.

Croatian often has separate masculine and feminine nouns for people:

• klijent = male client / client in a general masculine form
• klijentica = female client

Since the speaker is talking about a woman, klijentica is the natural choice.

Kasni is the 3rd person singular present tense of kasniti, which means to be late or to run late.

So:

• ja kasnim = I am late
• ti kasniš = you are late
• ona kasni = she is late

In Croatian, the present tense often covers what English expresses with is late or sometimes is running late.

Because na here expresses movement toward an event/place, so it takes the accusative case.

• na sastanak = to a meeting / for the meeting
• na sastanku = at the meeting

This is a very common contrast in Croatian:

• Idem na sastanak. = I’m going to a meeting.
• Sam sam na sastanku. = I’m at the meeting.

In your sentence, the person is late for/to the meeting, so na sastanak is correct.

Good question. Sastanak is a masculine inanimate noun, and in Croatian, many masculine inanimate nouns have the same form in nominative and accusative singular.

So:

• nominative: sastanak
• accusative: sastanak

This is normal.

Compare that with a masculine animate noun, where the accusative usually changes:

• klijent → klijenta

Je javila is the perfect tense of javiti, meaning she informed, she let someone know, or she notified.

So javila je / je javila means that she already performed that action in the past.

In this sentence, it means she has already sent a message or informed someone that she is coming.

Because in the Croatian past tense, the main participle agrees with the subject’s gender.

The subject is moja klijentica, which is feminine, so the form is:

• javio = masculine
• javila = feminine
• javilo = neuter

So:

• Moj klijent je javio... = My male client informed...
• Moja klijentica je javila... = My female client informed...

Because je is a clitic. In Croatian, clitics usually go in an early position in the clause, often right after the first element.

So ali je javila is the normal, neutral order.

• ali je javila = neutral, standard
• ali javila je = possible in some contexts, but more marked or stylistic

This early placement of short words like je, sam, se, ga, and so on is a very important feature of Croatian word order.

After verbs of saying, informing, knowing, thinking, and similar meanings, Croatian often uses da + a finite verb.

So:

• javila da dolazi = she informed that she is coming

This is how Croatian commonly introduces reported information.

English often uses:

• that she is coming

Croatian does the same kind of thing with da.

Yes. In Croatian, the present tense can sometimes refer to a current process or a near/future planned action, depending on context.

So da dolazi can mean:

• that she is coming
• that she’ll come
• that she’s on her way

In this sentence, because she is late, the idea is probably:

• she is still coming
• she has not cancelled
• she let them know she will arrive

Because Croatian is a pro-drop language. That means subject pronouns are often omitted when the meaning is already clear.

Here, the subject is already clear from:

• moja klijentica
• the verb forms
• the context

So ona is unnecessary.

You could add ona for emphasis, but normally you would not.

Because Croatian does not have articles like a and the.

So Croatian simply says:

• moja klijentica
• na sastanak

Whether something is a meeting, the meeting, or this meeting is understood from context, or made clear with other words if needed.

For example:

• ovaj sastanak = this meeting
• taj sastanak = that meeting

But very often, no article-like word is needed at all.

Not completely. Croatian word order is fairly flexible, but different orders sound more or less natural depending on emphasis.

The given sentence is a very natural neutral order:

Moja klijentica kasni na sastanak, ali je javila da dolazi.

You can move things around for emphasis, but then the focus changes. For learners, the given order is the best one to use as a model.