U ovom stupcu treba napisati prezent, a u onom stupcu perfekt.

Treba napisati is an impersonal way to say it is necessary to write, one should write, or in classroom/instruction language, simply write.

Croatian often uses treba + infinitive without naming the person when the instruction is general:

• Treba učiti. = One should study / It is necessary to study.
• Treba odgovoriti na pitanje. = The question should be answered.

In your sentence, the person is understood from context: this is an instruction to the learner.

So:

• U ovom stupcu treba napisati prezent
  = In this column, the present tense should be written = more naturally in English, Write the present tense in this column

This is about aspect, which is very important in Croatian.

• pisati = to write (imperfective: focusing on the process, repetition, or ongoing action)
• napisati = to write down / to write completely (perfective: focusing on completion)

In this sentence, the idea is not just be in the act of writing, but put the correct form into the column. That is a completed task, so napisati is natural.

Compare:

• Treba pisati uredno. = One should write neatly.
  (general manner/process)
• Treba napisati prezent. = One should write the present tense form.
  (complete the entry)

After u meaning in, Croatian usually takes the locative case.

So:

• u ovom stupcu = in this column
• u onom stupcu = in that column

The noun stupac changes to stupcu in the locative singular.

Basic pattern:

• nominative: ovaj stupac = this column
• locative: u ovom stupcu = in this column

Likewise:

• nominative: onaj stupac = that column
• locative: u onom stupcu = in that column

They are forms of demonstratives:

• ovaj = this
• onaj = that

In this sentence they appear in the locative singular masculine/neuter form because they go with u ... stupcu:

• ovaj → u ovom
• onaj → u onom

So:

• u ovom stupcu = in this column
• u onom stupcu = in that column

This is similar to how adjectives change to match the noun in case, number, and gender.

The dictionary form is stupac (column). When it declines, the stem changes:

• nominative singular: stupac
• locative singular: stupcu
• dative singular: stupcu

This kind of change is normal for some masculine nouns ending in -ac. You just have to learn the declined form.

So although the base form is stupac, after u meaning in, you get:

• u stupcu = in the column

Both can mean something like and, but they are not used in exactly the same way.

• i = simple and
• a = and / while / whereas / and in the other case

In this sentence, a contrasts the two columns:

• u ovom stupcu ...
• a u onom stupcu ...

So the meaning is closer to:

• In this column write the present tense, and in that column the perfect
• or even In this column write the present tense, whereas in that column write the perfect

Using a makes the contrast between this and that very natural.

They are the direct objects of napisati, and in this context they are masculine inanimate nouns whose accusative singular is the same as the nominative singular.

So:

• nominative: prezent

• accusative: prezent

• nominative: perfekt
• accusative: perfekt

That is why they look unchanged.

In other words:

• napisati prezent = to write the present tense
• napisati perfekt = to write the perfect tense

They are names of verb tenses:

• prezent = present tense
• perfekt = perfect (the common past tense in modern Croatian)

In school instructions, Croatian often uses just the tense name by itself:

• Napiši prezent glagola biti. = Write the present tense of the verb to be.
• Napiši perfekt. = Write the perfect tense.

So in your sentence, the full idea is something like:

• In this column, write the present-tense form
• and in that column, write the perfect-tense form

Yes. The verb phrase is understood for both halves of the sentence.

Full version:

• U ovom stupcu treba napisati prezent, a u onom stupcu treba napisati perfekt.

But Croatian often omits repeated material when it is obvious. So the shorter sentence is more natural:

• U ovom stupcu treba napisati prezent, a u onom stupcu perfekt.

English does this too:

• In this column, write the present tense, and in that one, the perfect.

Yes, Croatian word order is fairly flexible, though not completely free. The given sentence is natural and clear for instructions.

Possible variants include:

• Treba napisati prezent u ovom stupcu, a perfekt u onom stupcu.
• U ovom stupcu napišite prezent, a u onom perfekt.
  (direct command, plural/polite)
• U ovaj stupac napišite prezent, a u onaj perfekt.
  (with motion into the column rather than location in the column)

The original version sounds like a neutral written instruction.

That depends on whether Croatian sees the action as location or movement toward a destination.

• u + locative = in (location)
• u + accusative = into (motion/direction)

So:

• u ovom stupcu = in this column
• u ovaj stupac = into this column

In real classroom instructions, both types can appear depending on how the speaker imagines the task:

• U ovom stupcu treba napisati prezent.
  = focus on where the answer belongs
• U ovaj stupac napiši prezent.
  = focus on putting it into that space

Your sentence uses the location idea.

Croatian has no articles like English a/an/the.

Whether something is definite or indefinite is usually understood from context, word order, or demonstratives such as:

• ovaj = this
• onaj = that

So:

• stupac can mean a column or the column
• ovaj stupac clearly means this column
• prezent here naturally means the present tense because the context is grammatical instruction

English needs articles; Croatian does not.