U jednoj rubrici već je napisano "prezent", a u drugoj još treba dodati "perfekt".

Because after u meaning in for location, Croatian normally uses the locative case.

• base form: rubrika
• locative singular: rubrici

Since rubrika is feminine singular, the adjective/determiner must match it:

• jedna rubrika = one column/section/field
• u jednoj rubrici = in one column/section/field

So both words change:

• jedna → jednoj
• rubrika → rubrici

Here rubrika means something like:

• column
• field
• section
• entry space

The exact English word depends on context. If this is about a worksheet, chart, grammar table, or form, field or column is often the best choice.

They are agreeing with the feminine singular noun rubrici in the locative.

Croatian adjectives and adjective-like words must agree with the noun in:

• gender
• number
• case

So here:

• u jednoj rubrici = in one field
• u drugoj rubrici = in the other/second field

Because rubrika is feminine singular and is in the locative, both jednoj and drugoj are also feminine singular locative.

Because Croatian, like English, often leaves out a noun if it is already obvious.

So:

• u drugoj rubrici is the full form
• u drugoj is a shortened form meaning in the other one

This is very natural and common. English does the same:

• in one column ... in the other

They create a contrast:

• već = already
• još = still / yet

So the sentence contrasts two situations:

• in one field, prezent is already written
• in the other, perfekt still needs to be added

This pairing is very common in Croatian:

• već for something completed earlier than expected
• još for something not completed yet

This is a very good question, because learners often expect agreement with prezent, which is a masculine noun.

Here napisano is natural because the thing being written is treated as quoted content or a label, not so much as an ordinary masculine noun participating normally in the sentence. Croatian often uses neuter singular in this kind of impersonal/resultative wording with quoted words.

So već je napisano prezent or, with quotation marks, već je napisano ... is like saying:

• it is already written
• the label/content is already written there

If you treated prezent more like an ordinary noun as the grammatical subject, you might expect napisan, but with a quoted term or label, napisano is very idiomatic.

Yes, effectively it is.

It is built from:

• je = is
• napisano = written

So it functions like:

• is written
• has been written / is already written, depending on context

In this sentence it has a resultative feel: the word is already there on the page/in the field.

Treba + infinitive is a very common impersonal construction meaning:

• it is necessary to add
• one needs to add
• it still needs to be added

So:

• još treba dodati = still needs to be added

There is no specific subject like I, you, or we. Croatian often uses this impersonal structure when the person responsible is not important or is obvious from context.

Because dodati is the perfective verb, and the sentence is about a single complete action: adding perfekt to the other field.

• dodati = to add, as a completed act
• dodavati = to be adding / to add repeatedly / habitually

Here the idea is not repeated or ongoing addition. It is one needed step, so dodati is the natural choice.

Because a often connects two clauses that are parallel but contrasted.

Here the sentence sets up a comparison:

• in one field, one thing is already done
• in the other field, another thing still needs to be done

So a is better than i because it means something like:

• and meanwhile
• whereas
• while
• but in the other case

It is a very common Croatian connector for this kind of side-by-side contrast.

Because here they are being used as labels or grammar terms written into fields, not heavily integrated into the sentence as ordinary declined nouns.

When Croatian refers to a word as a word, especially as written content, it often leaves it in its citation form. That is why prezent and perfekt appear just as the labels themselves.

In other contexts, these nouns can of course decline normally.

Yes, Croatian word order is fairly flexible, though not completely free.

This sentence could be rearranged somewhat for emphasis, for example:

• U jednoj rubrici je već napisano ...
• Već je u jednoj rubrici napisano ...

But the original version sounds natural and clear. The order helps highlight the contrast:

• location 1
• what is already there
• location 2
• what still needs to be added

So even though other orders are possible, the given one is very idiomatic.

Because the Croatian sentence focuses on the state of the fields, not on the person doing the writing.

The important information is:

• one field already has prezent
• the other still needs perfekt

That is why Croatian uses forms like je napisano and treba dodati instead of naming an agent such as the teacher wrote or you should add. English often does something similar:

• prezent is already written
• perfekt still needs to be added