Kad kliknem na zadnji slajd, projektor ponekad prestane raditi.

Croatian usually drops subject pronouns when the verb ending already shows who is doing the action.

• kliknem = I click
• the -em ending already tells you it is 1st person singular

So Kad kliknem... naturally means When I click...
You could add ja only for emphasis or contrast:

• Kad ja kliknem... = When I click... / When I’m the one who clicks...

Kliknem is the 1st person singular present tense form of the perfective verb kliknuti.

That may seem strange at first, because in Croatian a perfective present often does not mean a simple present action in the same way English does. In sentences with kad and similar time words, it often refers to:

• a future action: Kad kliknem... = When I click...
• or a repeated trigger: Whenever I click...

So here it fits the idea that clicking on the last slide triggers what happens next.

This is mainly about aspect.

• kliknuti → perfective → one complete click
• klikati → imperfective → clicking repeatedly / in progress / habitually

In this sentence, the idea is that one completed action causes the next event:

• Kad kliknem na zadnji slajd... = when I click onto the last slide / when I make that click

If you said Kad klikam..., it would sound more like:

• while I am clicking
• when I keep clicking repeatedly

That is a different meaning.

Yes, kad and kada both mean when.

The difference is mostly style:

• kad = more common in everyday speech
• kada = a bit more formal or careful

So both are possible:

• Kad kliknem na zadnji slajd...
• Kada kliknem na zadnji slajd...

The version with kad sounds very natural and normal.

Because after kliknuti na, Croatian normally uses na + accusative for the thing you click on.

So:

• na zadnji slajd = on the last slide

Here, slajd is a masculine inanimate noun, and in the accusative singular it looks the same as the nominative:

• nominative: zadnji slajd
• accusative: zadnji slajd

By contrast, na zadnjem slajdu is locative, and it means on the last slide in the sense of location, not as the target of clicking.

So:

• kliknem na zadnji slajd = I click on the last slide
• tekst je na zadnjem slajdu = the text is on the last slide

Yes. In this sentence, zadnji slajd means the last slide.

A useful note, though:

• zadnji can mean last, rear, or back
• posljednji also means last/final

In everyday Croatian, people often use zadnji and posljednji almost interchangeably for last.
So you may also hear:

• na posljednji slajd

That would also be correct here.

Because Croatian often uses prestati + infinitive to mean stop doing something.

Here:

• prestane = stops
• raditi = to work

Together:

• prestane raditi = stops working

This is a very common structure:

• prestao sam govoriti = I stopped talking
• prestala je padati kiša = the rain stopped falling

So the projector is the subject, and raditi explains what it stops doing.

Because after prestati, Croatian usually uses the infinitive.

So:

• prestati raditi = to stop working
• prestati govoriti = to stop speaking
• prestati učiti = to stop studying

That is the standard pattern.
You do not need another finite verb there.

Ponekad means sometimes. It shows that this does not happen every time.

So the meaning is:

• when I click on the last slide, the projector sometimes stops working

Its position is fairly flexible. For example, these are all possible:

• Kad kliknem na zadnji slajd, projektor ponekad prestane raditi.
• Kad kliknem na zadnji slajd, ponekad projektor prestane raditi.
• Projektor ponekad prestane raditi kad kliknem na zadnji slajd.

The original word order is very natural.

Again, this is about aspect.

• prestati → perfective
• prestajati / prestati in present-like usage can express different nuances, but here prestane is the natural event form

Prestane raditi suggests a complete event:

• the projector cuts out
• it stops functioning

That fits well with ponekad, because the sentence describes an occasional event:

• sometimes it stops working

If you used prestaje raditi, it would sound more like an ongoing or descriptive process, which is less natural here.

Because Kad kliknem na zadnji slajd is a subordinate clause placed before the main clause.

Structure:

• subordinate time clause: Kad kliknem na zadnji slajd
• main clause: projektor ponekad prestane raditi

When that time clause comes first, Croatian normally separates it from the main clause with a comma.

So:

• Kad kliknem na zadnji slajd, projektor ponekad prestane raditi.

That comma is standard and expected here.