Nakon nevremena potok je postao mutan i brz.

Because nakon requires the genitive case, and the noun’s dictionary form is nevrijeme. Its genitive singular is nevremena.

So:

• nevrijeme = nominative singular
• nevremena = genitive singular

This noun belongs to the vrijeme type, where the form changes more than an English speaker might expect.

Nakon means after, and it takes the genitive.

So:

• nakon nevremena = after the storm / after the bad weather

This is something you simply need to memorize with the preposition:

• nakon + genitive

It is the perfect tense, which is the normal past tense used in everyday Croatian.

It is made from:

• je = auxiliary from biti (to be)
• postao = past participle of postati (to become)

So potok je postao means the stream became / has become.

Because postao agrees with the subject potok, which is masculine singular.

Croatian past participles agree in gender and number with the subject:

• potok je postao = masculine singular
• rijeka je postala = feminine singular
• jezero je postalo = neuter singular

So the ending tells you something about the noun it refers to.

Because they are describing the subject potok, so they must be adjectives agreeing with it in gender, number, and case.

• potok = masculine singular
• therefore: mutan and brz

If you used mutno or brzo, those would normally be adverb forms, not the right choice here.

After verbs like postati (to become), Croatian normally uses adjectives that agree with the subject:

• postao mutan i brz = became muddy and fast/swift

Yes. Brz literally means fast, and with water it means swift or fast-flowing.

So potok je postao brz means the stream started flowing quickly, which makes sense after a storm.

Depending on context, Croatian could also use words like nabujan (swollen) if the idea is rising water, but brz is completely correct here.

Because Croatian has no articles like English a/an/the.

So potok can mean:

• a stream
• the stream

The exact meaning depends on context. Croatian uses word order, context, and sometimes demonstratives like taj (that) if it wants to be more specific.

Yes. Croatian word order is fairly flexible.

This sentence is natural:

• Nakon nevremena potok je postao mutan i brz.

Another natural version would be:

• Potok je nakon nevremena postao mutan i brz.

The meaning stays basically the same, but the emphasis changes a little. Croatian also has special placement rules for short words like je, so learners will often see it near the beginning of the clause.

Because a short introductory phrase like Nakon nevremena usually does not need a comma in Croatian.

So the normal version is:

• Nakon nevremena potok je postao mutan i brz.

A comma might appear for style, pause, or emphasis, but it is not necessary here.