Kad je velika gužva, ljudi se kreću polako.

Kad means when.

In this sentence, Kad je velika gužva means When there is a big crowd / when it’s very crowded / when there is heavy congestion.

A few useful notes:

• kad is very common in everyday Croatian
• you may also see kada, which means the same thing but sounds a bit more formal or full
• in casual speech, kad is usually preferred

Je is the 3rd person singular present of biti = to be.

So:

• je = is
• Kad je velika gužva = literally When is a big crowd
• more natural English: When there is a big crowd or When it’s very crowded

Croatian often uses to be in places where English uses there is/there are or a different structure.

Gužva is a very common Croatian noun that can mean:

• crowd
• congestion
• rush
• jam, depending on context

So velika gužva can suggest:

• a lot of people in one place
• traffic-like congestion
• a situation where movement is difficult because it’s crowded

In this sentence, it suggests that there are so many people that moving becomes slow.

Because gužva is a feminine singular noun, and adjectives must agree with the noun in:

• gender
• number
• case

So:

• velik = basic dictionary form of the adjective
• velika = feminine singular form
• gužva = feminine singular

That is why you get velika gužva.

Ljudi means people.

Its singular is usually čovjek = person / man.

This is not a regular plural pattern, so it is something you simply learn as a vocabulary item:

• čovjek = person, man
• ljudi = people

Because ljudi is plural, the verb must also be plural:

• ljudi se kreću = people move

Kreću se means move or are moving around.

It comes from the verb kretati se, which is a reflexive verb. The basic idea is movement:

• kretati se = to move, to be in motion
• ljudi se kreću = people move / people are moving

In this sentence, it refers to physical movement through a crowded space.

In this sentence, se is necessary because the verb is kretati se, not just kretati.

• kretati se = to move
• kretati without se usually means to start, to set in motion, or to move something, depending on context

So:

• Ljudi se kreću = People move
• Ljudi kreću would usually sound like People are setting off / starting to go

That is a different meaning.

Both are possible in Croatian, but se is a clitic, and clitics usually like to stand near the beginning of the clause, often in second position.

So:

• Ljudi se kreću polako. = very natural
• Ljudi kreću se polako. = not natural in standard usage
• Kreću se polako. = also natural, if the subject is omitted or understood

In short:

• se cannot just go anywhere
• clitic placement follows special word-order rules in Croatian

Kreću is the 3rd person plural present tense form.

The subject is ljudi = people, which is plural, so the verb must also be plural.

So:

• ja se krećem = I move
• ti se krećeš = you move
• on/ona se kreće = he/she moves
• mi se krećemo = we move
• vi se krećete = you all move
• oni/one se kreću = they move

Here, ljudi matches the plural form kreću.

Polako means slowly here.

So:

• ljudi se kreću polako = people move slowly

But polako is a flexible word in Croatian and can also mean things like:

• take it easy
• carefully
• not so fast

For example:

• Polako! = Slow down! / Easy!

In this sentence, it is simply an adverb describing how people move.

Because it modifies the verb kreću se (move), so Croatian uses an adverb, not an adjective.

Compare:

• spor čovjek = a slow person
  (spor is an adjective)
• kreću se polako = they move slowly
  (polako is an adverb)

English works the same way:

• slow person
• move slowly

Because the sentence expresses a general fact or habitual situation, not one specific event.

So Croatian uses the present tense:

• Kad je velika gužva, ljudi se kreću polako.
• When there is a big crowd, people move slowly.

This is the same idea as in English:

• not just right now
• but in general, whenever this situation happens

Because Kad je velika gužva is a subordinate clause placed before the main clause.

Structure:

• Kad je velika gužva = dependent clause
• ljudi se kreću polako = main clause

Croatian normally uses a comma between them when the kad clause comes first.

So the comma here is standard and correct.

Croatian word order is fairly flexible, but not completely free.

The normal version here is:

• Kad je velika gužva, ljudi se kreću polako.

You may also hear slight variations, for example:

• Kad je velika gužva, ljudi se polako kreću.

That still means the same thing, but the focus may shift a little.

What you usually should not change casually is the position of clitics like se and je, because they follow special placement rules.

So:

• some movement of words is possible
• clitics are much less flexible

Yes, depending on context, kad here can feel like when or whenever.

Because the sentence expresses a repeated general situation, an English translation like this is also possible:

• Whenever there is a big crowd, people move slowly.

So even though kad literally means when, the sense can sometimes be broader in English.