U desnom stupcu sam pogriješila prezent, ali sam u lijevoj rubrici točno napisala infinitiv.

Those are feminine singular past participle forms. In Croatian past tense, the participle agrees with the speaker's gender and number.

So:

• female speaker: pogriješila, napisala
• male speaker: pogriješio, napisao

A man would say: U desnom stupcu sam pogriješio prezent, ali sam u lijevoj rubrici točno napisao infinitiv.

Croatian past tense is usually made with:

• the present tense of biti = to be
• plus the past participle

So here:

• sam pogriješila = I made a mistake / I got it wrong
• sam napisala = I wrote

The word sam is the 1st person singular form of biti in this tense.

Because sam is a clitic. Croatian clitics usually go near the beginning of the clause, typically in second position.

So:

• U desnom stupcu sam pogriješila...
• ali sam u lijevoj rubrici...

This is normal Croatian word order. English speakers often expect the verb to come first, but Croatian clitics follow their own placement rules.

Both phrases use u to mean in, expressing location, so they take the locative case.

• u desnom stupcu = in the right column
• u lijevoj rubrici = in the left box/field/column

If u meant movement into something, Croatian would normally use the accusative instead. Here it is static location, so locative is used.

Because adjectives must agree with the noun they describe in gender, number, and case.

• stupac is masculine singular, so in the locative it becomes stupcu, and the adjective becomes desnom
• rubrika is feminine singular, so in the locative it becomes rubrici, and the adjective becomes lijevoj

So the endings are different because the nouns are different.

They are in the accusative, because they are direct objects of the verbs:

• pogriješila prezent
• napisala infinitiv

For many masculine inanimate nouns in Croatian, the accusative singular looks the same as the nominative singular. That is why you see:

• prezent
• infinitiv

with no visible change.

In this sentence, pogriješiti nešto means to get something wrong. So prezent is the thing that was done incorrectly.

Here prezent does not mean the present in a general time sense. It means the present-tense form or present tense in a grammar exercise.

So the structure is basically:

• pogriješiti prezent = to get the present tense wrong

Because točno is an adverb, and it modifies the verb napisala.

• točno napisala = wrote correctly

If you used točan, that would be an adjective describing a noun:

• točan infinitiv = the correct infinitive

So:

• točno = correctly
• točan / točna / točno = correct

They are similar, but not exactly the same.

• stupac usually means a column, especially in a table
• rubrika often means a box, field, entry, heading, or section

In schoolwork, forms, and tables, rubrika can sometimes overlap with column, but it often suggests a labeled space where something is written. That is why both words can make sense in a sentence like this.

Because the sentence talks about completed actions with clear results:

• the speaker got one form wrong
• the speaker wrote the other form correctly

Croatian often uses perfective verbs for single completed actions:

• pogriješiti = to make a mistake / get something wrong
• napisati = to write down, finish writing

If you used imperfective forms, it would sound more ongoing, habitual, or less focused on the completed result.

Because ali means but and connects two clauses in contrast. In standard Croatian, a comma is normally used before ali in this kind of sentence.

So the structure is:

• first clause
• comma
• ali
• second clause

That punctuation is completely normal here.