Learn Hardcore Croatian: Ova knjiga je debela, ali je papir u njoj vrlo tanak. - This book is thick, but the paper in it is very thin.

Knjiga (book) is a feminine noun, so you must say ova knjiga.
Ovo is used either as a neuter pronoun (this thing) or with neuter nouns, not with feminine nouns.

Why is it je debela and not just debela? Is je always needed?

Je is the present tense of biti (to be) for the 3rd person singular: on/ona/ono je = “he/she/it is”.

In standard Croatian, you normally use je in sentences like this:

Ova knjiga je debela. – This book is thick.

You can drop je only in very informal, colloquial speech or in certain fixed patterns (e.g. headlines, slogans), but learners should keep je. So:

Correct, standard: Ova knjiga je debela.
Colloquial / non‑standard: Ova knjiga debela. (sounds incomplete or dialectal)

Why does the sentence repeat je: je debela, ali je papir u njoj vrlo tanak? Could you leave out the second je?

Croatian typically repeats the verb biti in both clauses:

Ova knjiga je debela, ali je papir u njoj vrlo tanak.

This is natural and completely standard.

You can omit the second je in more casual speech:

Ova knjiga je debela, ali papir u njoj vrlo tanak.

…but that sounds more colloquial and a bit “compressed”. As a learner, you’re safer and more natural if you keep je in both parts.

What does debela literally mean? Does it mean “fat” or “thick”?

The adjective debeo / debela / debelo literally means fat or thick and is used for both:

fat person/animal: debela mačka – a fat cat
thick object: debela knjiga – a thick book, debeli zid – a thick wall

In English you avoid saying “fat book”, but in Croatian debela knjiga is perfectly normal and doesn’t sound rude; it just means the book has many pages or a thick spine.

Why is debela ending in ‑a, but tanak ends in ‑ak here?

Two different adjective forms are used, because they agree with different nouns:

Ova knjiga je debela.
- knjiga – feminine singular
- adjective: debeo → feminine form debela
… ali je papir u njoj vrlo tanak.
- papir – masculine singular
- adjective: base form tanak is already masculine singular

So:

feminine singular: debela knjiga
masculine singular: tanak papir

If you described a feminine noun with tanak, you’d change it:

tanka bilježnica – a thin notebook (feminine noun)

What is the function of u in u njoj? Why not just njoj?

U is a preposition meaning in / inside. It must be used with a noun or pronoun if you want to say in it:

u knjizi – in the book
u njoj – in her / in it (referring to a feminine noun, here: knjiga)

Njoj on its own means “to her / for her / in her” depending on context, but here you specifically need the preposition u to give the sense of inside. So:

u njoj = in it (in the book)

Why is it u njoj and not u nje or u joj?

Croatian personal pronouns change forms by case. For ona (she), the relevant forms are:

Nominative: ona – she
Genitive: nje – of her
Dative/Locative: njoj – to her / at/in her
Short clitic dative: joj – to her

After the preposition u meaning “in/inside”, when it denotes location, you need the locative case. For ona, the locative singular is njoj:

u njoj – in her / in it (feminine noun)

So:

u nje – wrong for location here
u joj – wrong; joj is a clitic, and after u you use the full form njoj.

What does u njoj refer to exactly? How do we know it means “in the book”?

Njoj is a 3rd person singular feminine pronoun in dative/locative, so it refers back to the last relevant feminine singular noun in the context.

In this sentence, the obvious feminine singular noun is knjiga:

Ova knjiga je debela, ali je papir u njoj vrlo tanak.
→ This book is thick, but the paper in it is very thin.

So u njoj = u knjizi (in the book). The reference is clear from context and gender agreement.

Why is it vrlo tanak and not jako tanak or veoma tanak? Are they different?

All three mean roughly very thin:

vrlo tanak – very thin (more neutral, slightly more formal)
jako tanak – very thin / really thin (very common in speech)
veoma tanak – very thin (also correct, sounds a bit bookish or formal to some)

In this sentence, vrlo tanak is just a natural, neutral choice. You could say:

… ali je papir u njoj jako tanak.
… ali je papir u njoj veoma tanak.

All are grammatical; the difference is mostly in style/register.

Could the word order be “ali papir u njoj je vrlo tanak”? Is there a difference?

Yes, you can say:

Ova knjiga je debela, ali papir u njoj je vrlo tanak.

This is also correct. The main difference is emphasis and rhythm:

… ali je papir u njoj vrlo tanak.
→ neutral, fairly typical; je comes early in the clause (as a clitic verb), which is common.
… ali papir u njoj je vrlo tanak.
→ puts a bit more emphasis on papir u njoj as a block (“but the paper in it is very thin”).

Both are fine. The first is slightly more “textbook neutral” in terms of clitic position.

Why is it ova knjiga je debela and not ova je knjiga debela? Is that wrong?

Both are possible:

Ova knjiga je debela. – neutral sentence.
Ova je knjiga debela. – also correct, but the focus shifts slightly.

In ova je knjiga debela, placing je after ova can give gentle emphasis to ova (“THIS book is thick…” as opposed to some other book). It’s not dramatic, but native speakers use word order this way to signal focus.

For a learner, Ova knjiga je debela is the safest default.

Why is knjiga feminine? Is there any way to tell from the ending?

Most nouns ending in ‑a in the nominative singular are feminine in Croatian:

žena – woman (feminine)
stolica – chair (feminine)
knjiga – book (feminine)

There are exceptions (some masculine nouns end in ‑a, like kolega – male colleague), but ‑a is a strong general signal for feminine nouns. That’s why the demonstrative and adjective agree as feminine:

ova knjiga
debela knjiga

Is ali the only way to say “but”? Can I use nego here?

Ali is the normal conjunction for but:

Ova knjiga je debela, ali je papir u njoj vrlo tanak.

Nego is also often translated as but, but it’s used mainly:

After a negation, meaning but rather / but instead
- Nije debela knjiga, nego tanak časopis.
  – It’s not a thick book, but rather a thin magazine.
In certain contrastive structures with comparisons.

In your sentence, you are just contrasting two facts (book thick vs paper thin), without a negation, so ali is the correct and normal choice, not nego.

If I say papir u njoj instead of papir u knjizi, is that more natural?

Both are natural, but they have slightly different feels:

papir u knjizi – explicit: the paper in the book
papir u njoj – shorter, avoids repeating knjiga, more natural in continuous speech.

Because knjiga was already mentioned right before, native speakers prefer u njoj to avoid repetition, exactly as in English we say “in it” instead of repeating “in the book” every time.