Na zaslonu vidim novu datoteku, ali ne mogu kliknuti na nju.

Because na can take different cases depending on meaning.

• na + locative = location, meaning on / at
• na + accusative = motion toward something, meaning onto / to

Here, the file is already located on the screen, so Croatian uses the locative:

• na zaslonu = on the screen

Compare:

• Datoteka je na zaslonu. = The file is on the screen.
• Stavio sam datoteku na zaslon. = I put the file onto the screen.

So zaslonu is the locative singular of zaslon.

Because vidjeti / vidim takes a direct object, and direct objects are usually in the accusative case.

The noun datoteka is feminine, so in the accusative singular it becomes:

• datoteka → datoteku

The adjective must match the noun in gender, number, and case:

• nova datoteka = a new file
• novu datoteku = a new file (as the direct object)

So:

• Vidim novu datoteku. = I see a new file.

Because Croatian adjectives have to agree with the noun they describe.

Here the noun is:

• datoteka — feminine singular

But it is also in the accusative, because it is the object of vidim.

So the adjective changes too:

• nominative: nova datoteka
• accusative: novu datoteku

This is a very common pattern in Croatian: adjectives change form depending on case.

Croatian often uses a modal verb + infinitive, just like English:

• mogu = I can
• kliknuti = to click

So:

• mogu kliknuti = I can click
• ne mogu kliknuti = I cannot click

The negation ne goes with the finite verb:

• mogu → ne mogu

This is normal Croatian word formation with modal verbs.

Because Croatian normally says kliknuti na something, not simply kliknuti something.

So the pattern is:

• kliknuti na + accusative

Examples:

• kliknuti na gumb = to click on the button
• kliknuti na poveznicu = to click on the link
• kliknuti na nju = to click on it

This is one of those verb-preposition combinations that you mostly have to learn as a unit:

• kliknuti na nešto = click on something

Because after a preposition, Croatian uses the full/stressed pronoun form, not the short clitic form.

For ona in the accusative:

• short form: ju
• full form: nju

After prepositions like na, u, za, o, you use the full form:

• na nju
• za nju
• u nju

So:

• correct: kliknuti na nju
• not correct here: kliknuti na ju

It refers back to datoteku.

Since datoteka is a feminine singular noun, the matching pronoun is feminine singular too:

• datoteka → nju in this sentence

So the second part avoids repeating the noun:

• ...ali ne mogu kliknuti na novu datoteku.
• more natural: ...ali ne mogu kliknuti na nju.

This is similar to English it, but Croatian pronouns still show gender and case.

This is mainly about aspect.

• kliknuti is perfective: a single completed click
• klikati is imperfective: clicking repeatedly or in a general ongoing sense

In this sentence, the meaning is that the speaker cannot perform the single action of clicking the file, so kliknuti is the natural choice.

Very roughly:

• Ne mogu kliknuti na nju. = I can't click on it. / I can't manage to click it.
• Ne mogu klikati na nju. = I can't keep clicking on it / I can't click on it repeatedly.

For a simple computer-action sentence, kliknuti is the usual choice.

Not completely. Croatian word order is more flexible than English, but some orders sound more natural than others.

The given sentence is very natural:

• Na zaslonu vidim novu datoteku, ali ne mogu kliknuti na nju.

You could also move things around for emphasis, for example:

• Novu datoteku vidim na zaslonu, ali ne mogu kliknuti na nju.

That puts more emphasis on novu datoteku.

However, Croatian still has rules about where certain short words and clitics go, and the most neutral word order is often the best one for learners. So the original sentence is a good standard model.

Normally, no—not in standard Croatian for this meaning.

That is because kliknuti usually goes with na + object, not with a direct object by itself:

• correct: kliknuti na nju
• not the normal pattern: kliknuti je

So unlike vidjeti je = see it, with kliknuti you usually keep the preposition:

• ne mogu kliknuti na nju

This is worth remembering as a fixed pattern:

• vidjeti nešto
• kliknuti na nešto

No. You may also hear ekran.

Both can mean screen, but there can be slight differences in style or preference:

• zaslon often sounds a bit more standard or technical
• ekran is also very common in everyday speech

So you may hear:

• Na zaslonu vidim...
• Na ekranu vidim...

Both are understandable. In your sentence, zaslonu is perfectly natural.