U prvoj rubrici piše ime, a u drugoj prezime.

Rubrica here means a box, field, or column on a form, table, or document.

So u prvoj rubrici means in the first field/box/column, and u drugoj rubrici means in the second field/box/column.

In forms and paperwork, rubrica is a very common word.

Because the preposition u often means in and here it is followed by the locative case.

So:

• u rubrici = in the field / in the box
• u prvoj rubrici = in the first field
• u drugoj rubrici = in the second field

This is a very common pattern in Croatian:

• u kući = in the house
• u školi = in school
• u rubrici = in the field/box

They are all in the locative singular feminine.

The noun rubrica is feminine, so the adjective/ordinal has to agree with it.

Base forms:

• prva rubrica = the first field
• druga rubrica = the second field

After u, they change to locative:

• u prvoj rubrici
• u drugoj rubrici

So:

• prva → prvoj
• druga → drugoj
• rubrica → rubrici

This is normal adjective-noun agreement in Croatian: the adjective must match the noun in gender, number, and case.

Piše is the 3rd person singular present of pisati, meaning to write.

But in this sentence it does not literally mean that someone is physically writing at this moment. In this kind of context, piše often means something like:

• you write
• one writes
• it goes
• it should be written

So U prvoj rubrici piše ime is understood as something like:

• In the first field, you write the first name
• The first name goes in the first field

This use is very common in instructions and form-filling language.

Croatian often omits repeated words when they are easy to understand from context.

So the full version would be:

• U prvoj rubrici piše ime, a u drugoj piše prezime.

But since the second piše is obvious, it is left out:

• U prvoj rubrici piše ime, a u drugoj prezime.

English does this too sometimes:

• In the first box, first name; in the second, surname.

So this is just a natural omission of repeated material.

Here they are functioning as the thing being written.

With pisati, that thing is normally in the accusative case. But both ime and prezime are neuter nouns, and in the singular their nominative and accusative forms are the same.

So:

• ime = nominative singular

• ime = accusative singular too

• prezime = nominative singular
• prezime = accusative singular too

That is why you do not see any visible change.

• ime = first name / given name
• prezime = surname / family name / last name

So on a form:

• Ime: John
• Prezime: Smith

A native English speaker may expect name to cover both meanings, but Croatian usually separates them clearly:

• ime
• prezime

A often translates as and, but it is not exactly the same as plain English and in every situation.

Here a links two contrasting or parallel pieces of information:

• in the first field ...
• in the second ...

So it feels like:

• and
• while
• whereas
• and in the other one...

In this sentence, a is a very natural connector because the speaker is comparing two fields.

It could in some contexts, but a is better here.

• i = simple and
• a = and / while / whereas, often with a contrast or parallel structure

Because the sentence sets up first field versus second field, a sounds more natural:

• U prvoj rubrici piše ime, a u drugoj prezime.

Using i would sound less elegant and less natural in this kind of comparison.

No, Croatian word order is fairly flexible, but the given version is very natural and clear.

The sentence starts with location:

• U prvoj rubrici = in the first field

Then comes the verb:

• piše

Then the thing written:

• ime

This ordering works well because it matches the practical instruction: first the place, then what goes there.

You may hear other versions in special contexts, but the original is the most natural neutral form for instructions like this.

Croatian has no articles like English a and the.

So Croatian simply says:

• u prvoj rubrici
• literally: in first field

But in English we usually need an article:

• in the first field

Croatian expresses definiteness through context, word order, and meaning, not with articles.

Most likely, yes.

Because of the word rubrica, the sentence sounds like an instruction related to:

• a form
• an application
• a questionnaire
• a table or document

So it naturally suggests:

• Write your first name in the first field, and your surname in the second.

That is why piše here is best understood as instructional, not as a literal statement about someone currently writing.