Kad pada jaka kiša, rijeka je mutna.

Kad means when here.

Yes, kad and kada both mean when. In everyday speech, kad is very common and slightly more conversational. Kada can sound a bit more formal or careful, but both are correct.

So you could also say:

Kada pada jaka kiša, rijeka je mutna.

The meaning stays the same.

Pada is the 3rd person singular present tense of the verb padati.

In this sentence, padati is used for weather, so kiša pada means rain is falling or simply it is raining.

Because the subject is jaka kiša (heavy rain), the verb has to be singular:

• kiša pada = the rain falls / it rains

Croatian often expresses weather more literally than English does.

Yes, literally jaka kiša is strong rain, but idiomatically it means heavy rain.

This is a very common pattern in Croatian:

• jak vjetar = strong wind
• jaka kiša = heavy rain

So even though English prefers heavy rain, Croatian naturally uses jak / jaka / jako for this idea.

Because adjectives in Croatian agree with the noun they describe.

Here, kiša is:

• feminine
• singular
• nominative

So the adjective must also be feminine singular nominative:

• masculine: jak
• feminine: jaka
• neuter: jako

That is why it is:

jaka kiša

not jak kiša or jako kiša.

Rijeka is the subject of the main clause, so it is in the nominative case.

The main clause is:

rijeka je mutna = the river is muddy

Since rijeka is a feminine singular noun in nominative, its basic dictionary form is already rijeka.

The same is true for kiša in the first clause:

• kiša pada → kiša is also the subject, so it is nominative.

Je is the 3rd person singular present tense of biti (to be).

Croatian usually needs the verb to be in sentences like:

• Rijeka je mutna. = The river is muddy.
• Voda je hladna. = The water is cold.

Unlike in some languages, you generally cannot leave it out in standard Croatian present-tense sentences like this.

So:

• rijeka mutna would sound incomplete or nonstandard here
• rijeka je mutna is correct

Mutna is the adjective muddied / murky / cloudy, and it agrees with rijeka.

Since rijeka is:

• feminine
• singular
• nominative

the adjective must also be:

• feminine
• singular
• nominative

So:

• masculine: mutan
• feminine: mutna
• neuter: mutno

That is why the sentence has:

rijeka je mutna

Croatian does not have articles like English a and the.

So:

• rijeka can mean a river or the river
• kiša can mean rain or the rain

Which one is meant depends on context.

In this sentence, English naturally uses the river, but Croatian simply says rijeka.

This is completely normal.

Because Kad pada jaka kiša is a subordinate clause placed before the main clause.

So the structure is:

Kad pada jaka kiša, | rijeka je mutna.

This is similar to English:

• When it rains heavily, the river is muddy.

In Croatian, a comma is normally used to separate this kind of introductory clause from the main clause.

Yes. Croatian word order is more flexible than English word order.

You can also say:

Rijeka je mutna kad pada jaka kiša.

This means the same thing.

The version with Kad... first puts the time/condition first and sounds very natural. The version with Rijeka je mutna... first puts the main statement first.

Both are correct.

Usually it expresses a general truth or a habitual situation:

Whenever / when heavy rain falls, the river is muddy.

The present tense in Croatian often works this way, especially with kad clauses.

Depending on context, it could also describe a current situation, but without more context, most learners should understand it as a general statement.

Yes, but it changes the nuance a little.

• Kad pada jaka kiša, rijeka je mutna.
  = When heavy rain falls, the river is muddy.

• Kada god pada jaka kiša, rijeka je mutna.
  = Whenever heavy rain falls, the river is muddy.

So kada god makes the repeated, every-time meaning more explicit. The original sentence already suggests that idea, but kada god stresses it more.

A simple approximate pronunciation is:

Kad PA-da YA-ka KEE-sha, RYEE-ye-ka ye MOOT-na.

A few helpful sound notes:

• j is pronounced like English y
• š sounds like sh
• je sounds roughly like ye
• rijeka is approximately rye-ka or rye-ye-ka, depending on the speaker

So:

• kiša ≈ KEE-sha
• rijeka ≈ RYEE-ka / RYEE-ye-ka
• mutna ≈ MOOT-na