Obično zakazujem sastanke početkom tjedna, ali sam danas zakazala pregled za četvrtak i odmah potvrdila termin.

Because the sentence talks about two different kinds of actions.

zakazujem is present tense and expresses a habitual/repeated action: I usually schedule meetings...

zakazala is part of the past tense expression sam zakazala and refers to one completed action today: I scheduled an appointment today...

There is also an aspect difference:

• zakazivati / zakazujem = imperfective, focusing on repetition or process
• zakazati / zakazala = perfective, focusing on a completed act

So Croatian is showing both:

• habit in the first clause
• completed event in the second

Obično means usually.

It is an adverb that modifies the whole action: Obično zakazujem sastanke... = I usually schedule meetings...

Putting it at the beginning is very natural and neutral. Croatian word order is flexible, so it can move, but the sentence sounds most natural with obično near the front when it sets the general pattern for the whole clause.

Because the past participle in Croatian agrees with the speaker’s gender.

• zakazala = a female speaker
• zakazao = a male speaker

So this sentence is being said by a woman.

The same applies to potvrdila:

• potvrdila = female speaker
• potvrdio = male speaker

Sam is the auxiliary verb used to form the Croatian perfect tense.

Croatian past tense is usually built like this:

subject + auxiliary of biti + past participle

So:

• sam zakazala = I scheduled
• literally something like am scheduled, though you should think of it simply as the normal past tense form

In first person singular, the auxiliary is sam.

This is why Croatian uses two words where English often uses only one:

• English: I scheduled
• Croatian: sam zakazala

Because Croatian often omits the repeated auxiliary when two past-tense verbs share the same subject and tense.

So this:

ali sam danas zakazala pregled za četvrtak i odmah potvrdila termin

really means:

ali sam danas zakazala pregled za četvrtak i odmah sam potvrdila termin

Both are possible, but leaving out the second sam is very natural and avoids repetition.

Because sastanke is the accusative plural, used for a direct object.

The verb zakazivati / zakazati takes a direct object: you schedule something.
Here, the thing being scheduled is meetings.

Forms of sastanak:

• nominative plural: sastanci
• accusative plural: sastanke

So:

• sastanci = meetings, as the subject
• zakazujem sastanke = I schedule meetings, as the object

Because pregled is singular masculine inanimate, and in that pattern the accusative singular is the same as the nominative singular.

So:

• nominative singular: pregled
• accusative singular: pregled

That is why zakazala pregled looks unchanged.

The same thing happens with termin here:

• nominative singular: termin
• accusative singular: termin

Početkom tjedna means at the beginning of the week or early in the week.

It is a very common expression.

Grammatically:

• početkom is the instrumental singular of početak = beginning
• tjedna is the genitive singular of tjedan = week

You do not need to build it word-by-word in normal use; it is best learned as a set phrase.

A close alternative is:

na početku tjedna

That also means at the beginning of the week, but početkom tjedna is a bit more compact and very common in everyday language.

Because za + accusative is often used for something that is scheduled/arranged for a certain time.

So:

pregled za četvrtak = an appointment/check-up for Thursday

This is a very natural way to talk about booking something for a particular day.

You may also see u četvrtak, which means on Thursday. The difference is roughly:

• za četvrtak = scheduled for Thursday
• u četvrtak = on Thursday

Also note that četvrtak does not visibly change here because it is masculine inanimate singular, and its accusative is the same as its nominative.

They are related, but not identical.

• pregled = the examination, check-up, or visit itself
• termin = the scheduled time slot or appointment time

So in this sentence:

• she scheduled the check-up: zakazala pregled
• then she confirmed the appointment time: potvrdila termin

In English, both can sometimes be translated as appointment, but Croatian often keeps this distinction clearer.

Because danas means today, but the action can still be completed earlier on the same day.

So:

danas sam zakazala... = I scheduled ... today

That is completely normal.
English works the same way:

• Today I booked an appointment
• Today I confirmed the time

So there is no conflict between today and past tense.

Yes, Croatian word order is flexible, but there are rules about short unstressed words like sam.

sam is a clitic, and clitics tend to go near the beginning of the clause.

That is why ali sam danas zakazala sounds natural.

You could also say:

ali danas sam zakazala pregled za četvrtak...

That version puts more emphasis on danas.

So the word order can change for focus and style, but the placement of clitics like sam is not completely free.