Learn Hardcore Croatian: U udžbeniku čitam odlomak o Zagrebu i jedan odlomak o moru. - In the textbook I read one paragraph about Zagreb and one paragraph about the sea.

u udžbeniku = in the textbook (location inside the book)
na udžbeniku would mean on the textbook (physically on its surface), which doesn’t fit here.
iz udžbenika (with genitive) means from the textbook and would be possible if you wanted to stress that the text comes from that book:
- Iz udžbenika čitam odlomak… = From the textbook I’m reading a passage…

So u udžbeniku is used because you are talking about reading something that is located in the textbook.

What case is udžbeniku, and how do I know it’s locative and not dative?

Formally, udžbeniku can be either dative singular or locative singular of udžbenik. In this sentence it is locative, because:

The preposition u meaning in always takes the locative case.
With u, na, o, etc., the preposition itself tells you which case is used, even if the form looks the same as another case.

So here: u + udžbeniku = preposition of place → locative.

Why is there no ja before čitam? Don’t we need the subject pronoun?

In Croatian, subject pronouns (like ja, I) are usually dropped, because the verb ending already shows the person:

čitam = I read / I am reading
čitaš = you read (singular)
čita = he / she / it reads

You would say Ja čitam… only if you want to emphasize I, for example in contrast to someone else:

Ja čitam, a ti pišeš. = I am reading, and you are writing.

In neutral sentences like this one, ja is normally omitted.

What exactly is the form čitam, and does it mean I read or I’m reading?

Čitam is:

Present tense
First person singular (1st sg)
Imperfective aspect
From the verb čitati (to read).

Because it is imperfective, Croatian present čitam can cover both English meanings:

I read (in general / habitually).
I am reading (right now).

Context decides which English translation is better. In this sentence, both readings are possible.

Why is odlomak in this form and not odlomka or odlomci?

Odlomak here is the direct object of čitam, so it is in the accusative singular. For masculine inanimate nouns like odlomak:

Nominative sg: odlomak
Accusative sg: odlomak (same as nominative)
Genitive sg: odlomka

That is why you see čitam odlomak and not čitam odlomka (which would be genitive) or čitam odlomci (nominative plural).

What does jedan odlomak add? Why is jedan only before the second odlomak?

Jedan literally means one, and here it works much like English one paragraph:

…čitam odlomak o Zagrebu i jedan odlomak o moru.
≈ …I’m reading a paragraph about Zagreb and one paragraph about the sea.

Only the second odlomak has jedan to highlight that there is one additional paragraph about the sea. You could also say:

čitam jedan odlomak o Zagrebu i jedan odlomak o moru

This would emphasize that you read one and one (one of each). Leaving jedan out in the first part is very natural and sounds less “counting-focused”.

Can I omit the second odlomak and say …čitam odlomak o Zagrebu i jedan o moru?

Yes, that is perfectly grammatical and quite natural:

U udžbeniku čitam odlomak o Zagrebu i jedan o moru.

Here jedan stands for jedan odlomak. The noun is understood from context, so it can be left out, just like in English: a paragraph about Zagreb and one about the sea.

Both versions (with or without repeating odlomak) are fine.

Why are o Zagrebu and o moru in those forms, and not o Zagreb or o more?

The preposition o meaning about takes the locative case in Croatian. So both Zagreb and more must be in the locative:

Zagreb → locative sg: Zagrebu
more (sea) → locative sg: moru

Patterns:

Many masculine nouns like Zagreb take -u in locative singular.
Neuter nouns in -e (like more) often change to -u in locative: more → moru.

So o Zagrebu, o moru are required by the preposition o.

Why is Zagrebu capitalized but moru is not?

Zagreb is a proper noun (name of a city), so Croatian capitalizes it, just like English:

Zagreb → Zagrebu (locative) → still capitalized.

More (sea) is a common noun, not a name, so it is not capitalized:

o moru = about the sea (general).

This is the same rule as in English: about Zagreb and about the sea.

What exactly does udžbenik mean, and how is it different from knjiga?

Udžbenik means textbook, a book designed for learning a subject in school or university.

Knjiga is the general word for book of any kind. Compare:

udžbenik engleskog = English textbook
knjiga o Zagrebu = a book about Zagreb (could be any type of book).

In this sentence, udžbenik tells you that the reading is from a school textbook, not just any book.

How do you pronounce the đ in udžbeniku?

The letter đ represents a single consonant, similar to the j sound in English judge, but a bit softer and more “polished.”

So udžbeniku is pronounced approximately:

u-dž-be-ni-ku
dž ≈ English j in jam or dg in bridge.

The ž part in the spelling dž is there to show this specific sound; you say it as one unit, not as separate d + ž.

Could I move u udžbeniku to the end: Čitam odlomak o Zagrebu i jedan odlomak o moru u udžbeniku?

You can say that, and it is grammatically correct, but the most natural version is usually with the place expression at the beginning:

U udžbeniku čitam odlomak o Zagrebu i jedan odlomak o moru.

Putting u udžbeniku at the end may sound a bit heavier and can create a slight ambiguity (it might seem to attach only to jedan odlomak o moru). Croatian word order is flexible, but the original order is clearer and more typical.