Kad vidim ovaj kolač, odmah poželim još jedan komad.

Kad means when in this sentence.

It introduces a time clause: Kad vidim ovaj kolač = When I see this cake.

In everyday Croatian, kad is very common. You may also see kada, which means the same thing but can sound a little more formal or more careful in style.

Because vidim is the 1st person singular present tense of vidjeti / viditi (to see).

The subject is understood from the verb ending, so Croatian does not need to say ja unless you want emphasis.

• vidim = I see
• vidiš = you see
• vidi = he/she/it sees

So Kad vidim... literally means When I see...

Because ovaj kolač is the direct object of vidim, and with this noun the accusative singular looks the same as the nominative singular.

• nominative: ovaj kolač = this cake
• accusative: ovaj kolač = this cake

This happens because kolač is an inanimate masculine noun. In Croatian, masculine inanimate nouns usually have the same form in nominative and accusative singular.

Compare:

• Vidim ovaj kolač. = I see this cake.
• Vidim ovog čovjeka. = I see this man.

With čovjek (an animate masculine noun), the accusative changes.

This is a very important question because it involves aspect and meaning nuance.

• želim = I want / I am wanting
• poželim = I suddenly feel the desire / I get the urge / I come to want

The prefix po- often gives the idea of something starting, arising, or happening as a single event. Here, poželim suggests that the desire appears at that moment.

So:

• Kad vidim ovaj kolač, odmah želim još jedan komad.
  = more like When I see this cake, I want another piece.
• Kad vidim ovaj kolač, odmah poželim još jedan komad.
  = more like When I see this cake, I immediately start wanting another piece / I instantly crave another piece.

The sentence sounds very natural with poželim because it captures that sudden reaction.

Yes. In Croatian, just like in English, the present tense can describe a habitual or general truth.

So this sentence does not only mean one specific occasion. It can mean something like:

Whenever I see this cake, I immediately want another piece.

That is a very normal use of the present tense in Croatian.

Odmah means immediately / right away.

It shows that the reaction happens without delay:

• Kad vidim ovaj kolač, odmah poželim...
• When I see this cake, I immediately want...

It is placed before poželim because it modifies that verb phrase.

Croatian word order is flexible, so you may also hear similar variants, but this placement is very natural.

Još jedan komad means another piece or more literally one more piece.

Breakdown:

• još = more / still / another, depending on context
• jedan = one
• komad = piece

So:

• još jedan komad = one more piece
• in natural English: another piece

This is a very common Croatian way to express another one.

Because the phrase is the direct object of poželim, and komad is a masculine inanimate noun.

So the accusative singular is the same as the nominative singular:

• nominative: jedan komad
• accusative: jedan komad

That is why the sentence has:

poželim još jedan komad

If the noun were masculine animate, the accusative would change form.

Because Kad vidim ovaj kolač is a subordinate clause introduced by kad.

Croatian normally separates this kind of introductory clause with a comma:

• Kad vidim ovaj kolač, odmah poželim još jedan komad.

This is similar to English writing:

• When I see this cake, I immediately want another piece.

If the main clause came first, the comma rules can vary depending on structure and style, but with an opening kad-clause, the comma here is standard.

The subject is not missing; it is simply understood from the verb form.

• vidim already means I see
• poželim already means I want / I get the urge

Croatian often leaves out subject pronouns when they are clear from the verb ending.

So Kad vidim ovaj kolač... is completely normal.

You could say Kad ja vidim ovaj kolač..., but that would add emphasis, something like:

When I see this cake...
with stress on I.

Yes. Kad and kada both mean when.

So you can say:

• Kad vidim ovaj kolač, odmah poželim još jedan komad.
• Kada vidim ovaj kolač, odmah poželim još jedan komad.

Both are correct. Kad is shorter and very common in everyday speech. Kada can sound a little fuller or slightly more formal, but in many contexts the difference is small.

Croatian word order is fairly flexible, but some versions sound more natural than others.

The original sentence is very natural:

Kad vidim ovaj kolač, odmah poželim još jedan komad.

You could also hear variants like:

• Kad vidim ovaj kolač, poželim odmah još jedan komad.
• Odmah poželim još jedan komad kad vidim ovaj kolač.

These are possible, but the original order is smooth and neutral.

In Croatian, word order often changes for:

• emphasis
• rhythm
• style
• focus

So the meaning may stay the same while the emphasis shifts slightly.

Usually yes, or at least more sudden and event-like.

Želim describes a state: I want.
Poželim describes the moment the desire appears: I suddenly want / I get the urge / I come to want.

In this sentence, poželim fits the idea of seeing the cake and immediately feeling temptation. So it often sounds more vivid and more natural than plain želim here.