Kad složim dokumente u fascikl, stol izgleda uredniji.

Kad means when. It introduces a time clause:

• Kad složim dokumente u fascikl = When I put/arrange the documents into the folder

In Croatian, kad is very common in everyday speech. You may also see kada, which is a slightly fuller form of the same word.

This is a very common Croatian aspect question.

• složiti = perfective verb
• slagati / slažem = imperfective verb

Here, složim is the 1st person singular present form of the perfective verb složiti.

Why perfective? Because the sentence focuses on the action as a completed whole: once I finish putting the documents into the folder, the desk looks tidier.

So:

• Kad složim dokumente... = When I finish arranging / once I put the documents...
• Kad slažem dokumente... would suggest while I am arranging the documents or a more ongoing process, which is not the idea here.

In form, složim is present tense, but with a perfective verb Croatian often uses the present form to talk about a future completed action in clauses with kad.

So this can feel like:

• When I put the documents into the folder, the desk looks tidier
• or Once I’ve put the documents into the folder, the desk looks tidier

Depending on context, it can also have a habitual sense:

• Whenever I put the documents into the folder, the desk looks tidier

This is normal Croatian usage.

Croatian usually drops subject pronouns when they are not needed, because the verb ending already tells you the person.

• složim = I arrange / I put / I finish arranging

So ja is unnecessary unless you want emphasis or contrast:

• Kad ja složim dokumente... = When I’m the one who arranges the documents...

Without ja, the sentence sounds natural and neutral.

Dokumente is the accusative plural of dokumenti.

It is in the accusative because it is the direct object of the verb složim:

• složim što? = what do I arrange/put?
• dokumente = the documents

So:

• nominative plural: dokumenti
• accusative plural: dokumente

This is exactly what you would expect for the thing being acted on.

Because Croatian uses different cases after u depending on whether you mean:

• movement into something
• or location inside something

Here, the documents are being put into the folder, so Croatian uses the accusative:

• u fascikl = into the folder

If you were talking about where something already is, you would use the locative:

• u fasciklu = in the folder

So compare:

• Stavljam dokumente u fascikl. = I am putting the documents into the folder.
• Dokumenti su u fasciklu. = The documents are in the folder.

Fascikl means folder, especially a file folder or document folder used for papers.

It is a masculine noun. In this sentence:

• dictionary form: fascikl
• after u with movement: u fascikl (accusative)

A learner may expect something like folder from English, but fascikl is a normal Croatian word for this kind of object.

Because stol is the subject of the main clause:

• stol izgleda uredniji = the desk looks tidier

The thing that looks tidy is the desk, so stol is nominative singular.

Yes. Uredniji is the comparative of uredan:

• uredan = tidy, neat
• uredniji = tidier, neater

So:

• stol izgleda uredniji = the desk looks tidier

The comparative agrees with stol, which is:

• masculine
• singular
• nominative

So the adjective is also masculine singular nominative: uredniji.

Because the comparison is implicit.

• uredniji already means tidier
• the sentence implies tidier than before

Croatian often uses the comparative without explicitly saying than before.

If you wanted to make the comparison explicit, you could say something like:

• Stol izgleda uredniji nego prije. = The desk looks tidier than before.

But in your sentence, that extra phrase is simply understood.

Izgleda is from izgledati, which means to look / to appear.

So:

• stol izgleda uredniji = the desk looks tidier

This is about appearance, not necessarily an objective statement like is tidier. Croatian often uses izgledati the same way English uses look.

Yes. Croatian word order is flexible, though not random.

This sentence starts with the time clause:

• Kad složim dokumente u fascikl, stol izgleda uredniji.

You could also say:

• Stol izgleda uredniji kad složim dokumente u fascikl.

Both are grammatical. The version with kad... first gives a natural background-then-result structure:

1. when I put the documents in the folder
2. the desk looks tidier

That is probably why this order was chosen.

It can suggest more than simple physical movement.

The verb složiti often means:

• to arrange
• to put in order
• to organize neatly
• to place together

So složim dokumente u fascikl is not just I throw the documents into a folder. It usually suggests that the documents are put away in an orderly way.

That fits well with the result:

• stol izgleda uredniji = the desk looks tidier

Yes.

Depending on context, it can mean either:

• When/once I put the documents into the folder, the desk looks tidier
  → one situation being described

or

• Whenever I put the documents into the folder, the desk looks tidier
  → a general repeated pattern

Croatian often leaves that distinction to context. English sometimes forces you to choose more clearly than Croatian does.