Moj brat često pomiješa pridjev i imenicu kad brzo piše.

Because brat is the subject of the sentence, so it is in the nominative singular. The possessive moj has to agree with it in gender, number, and case:

• moj = masculine singular nominative
• brat = masculine singular nominative

So moj brat means my brother.

A form like moga/mojega brata would be a different case, not the subject form.

Croatian does not have articles like English the and a/an.

So:

• brat can mean brother, a brother, or the brother
• pridjev can mean an adjective or the adjective
• imenicu can mean a noun or the noun

The exact meaning usually comes from context.

Both are in the accusative singular because they are the direct objects of pomiješa.

They look different because they belong to different noun types:

• pridjev is a masculine inanimate noun

  • nominative: pridjev
  • accusative: pridjev
    In masculine inanimate nouns, nominative and accusative are often the same.

• imenica is a feminine noun

  • nominative: imenica
  • accusative: imenicu

So the sentence has two objects joined by i = and: pridjev i imenicu.

Here it is understood as present tense: he mixes up / he ends up mixing up.

More specifically, it is the 3rd person singular present of the perfective verb pomiješati.

A tricky point: pomiješa can also look like an aorist form in older or literary Croatian, but in a normal modern sentence like this one, with često and kad brzo piše, learners should read it as a present-time meaning.

This is a question of aspect.

• miješati = imperfective
  This focuses on the process or general repeated action.
• pomiješati = perfective
  This focuses on the action as a completed mistake/result.

So:

• često miješa pridjev i imenicu = he often confuses/mixes them up in general
• često pomiješa pridjev i imenicu = he often ends up mixing them up; it emphasizes the completed slip

With često, both can be possible depending on nuance, but pomiješa sounds a bit more like he often makes that mistake.

In this sentence, they are adverbs, not adjectives.

• često = often
• brzo = quickly

They modify verbs:

• često pomiješa = often mixes up
• brzo piše = writes quickly

Compare:

• brz brat = a fast brother → adjective describing a noun
• brzo piše = writes quickly → adverb describing a verb

Croatian adverbs often look like the neuter form of an adjective, which is why brzo may look familiar if you know brz, brza, brzo.

Because Croatian normally drops subject pronouns when the verb ending already shows who the subject is.

• pomiješa = he/she mixes up
• piše = he/she writes

The context makes it clear that the subject is moj brat, so adding on is unnecessary.

You could add on for emphasis, but the neutral version usually leaves it out.

Yes.

• kad = when
• kada = when

They mean the same thing here. Kad is shorter and very common in everyday Croatian. Kada can sound a bit fuller or slightly more formal, depending on context.

So this sentence could also be:

Moj brat često pomiješa pridjev i imenicu kada brzo piše.

Because kad brzo piše is a subordinate clause, and Croatian normally separates it from the main clause with a comma.

So the sentence is divided like this:

• main clause: Moj brat često pomiješa pridjev i imenicu
• subordinate clause: kad brzo piše

This comma is more standard in Croatian than it often is in English.

Yes. Croatian word order is fairly flexible because case endings show grammatical roles.

The given order is natural and neutral:

Moj brat često pomiješa pridjev i imenicu kad brzo piše.

You could also say:

Kad brzo piše, moj brat često pomiješa pridjev i imenicu.

That means essentially the same thing, but it puts the when he writes quickly part first for emphasis or flow.

Even though Croatian allows movement, not every order sounds equally natural, so the original sentence is a good default model.