Ništa ne razumijem kad govoriš prebrzo.

Yes, from an English point of view it looks like a double negative, but in Croatian this is completely normal and required.

• ništa = nothing
• ne razumijem = I do not understand

In Croatian, words like ništa (nothing), nikad (never), nitko (no one) usually go together with a negative verb.

So:

• Ništa ne razumijem = I understand nothing / I don’t understand anything

You would not normally drop ne here.

Yes. That is also correct.

Both mean the same thing, but the word order changes the focus slightly:

• Ništa ne razumijem... puts nothing first, so it sounds a bit more emphatic.
• Ne razumijem ništa... is also very natural and may sound a little more neutral.

Croatian word order is more flexible than English word order, especially for emphasis.

Because Croatian often leaves subject pronouns out when the verb ending already shows who the subject is.

Here the verb forms tell you everything:

• razumijem = I understand
• govoriš = you speak

So Croatian does not need ja (I) or ti (you) unless you want extra emphasis or contrast.

For example:

• Ja ništa ne razumijem = I don’t understand anything
  This stresses I.

• Kad ti govoriš prebrzo... = When you speak too fast...
  This stresses you.

Razumijem is the 1st person singular present tense form of the verb razumjeti (to understand).

So:

• ja razumijem = I understand
• ti razumiješ = you understand
• on/ona razumije = he/she understands

In your sentence:

• ne razumijem = I don’t understand

Govoriš is the 2nd person singular present tense form of govoriti (to speak).

So:

• ja govorim = I speak
• ti govoriš = you speak
• on/ona govori = he/she speaks

Here:

• kad govoriš prebrzo = when you speak too fast

Because Croatian often uses the present tense for general, repeated, or current situations, just like English does in sentences such as:

• I can’t understand you when you speak too fast.

This sentence is not talking about one specific past event. It describes a general situation: whenever that person speaks too fast, the speaker understands nothing.

So kad govoriš prebrzo is exactly what you would expect here.

Yes, you could.

• kad and kada both mean when
• kad is shorter and very common in everyday speech
• kada can sound a little more formal, careful, or literary, depending on context

So both are correct:

• Ništa ne razumijem kad govoriš prebrzo.
• Ništa ne razumijem kada govoriš prebrzo.

In normal conversation, kad is extremely common.

Prebrzo means too fast.

It is built from:

• brzo = fast / quickly
• pre- = a prefix meaning too, excessively, overly

So:

• brzo = fast
• prebrzo = too fast

This is very common in Croatian:

• preskupo = too expensive
• prekasno = too late
• previše = too much

Here prebrzo is an adverb, describing how someone speaks.

Yes, ništa is the object of razumijem.

The verb razumjeti takes a direct object, so you would expect the accusative case. With ništa, the nominative and accusative forms are the same, so there is no visible ending change here.

So in this sentence:

• Ništa = direct object, meaning nothing

That is why the sentence literally works as:

• Nothing I do not understand when you speak too fast

but in natural English:

• I don’t understand anything when you speak too fast.

Yes, but it means something slightly different.

• Ništa ne razumijem kad govoriš prebrzo = I understand nothing when you speak too fast
• Ne razumijem te kad govoriš prebrzo = I don’t understand you when you speak too fast

The original sentence focuses on understanding nothing of what is being said in general.

The version with te focuses more directly on you as the person being understood.

Both are natural; the choice depends on what you want to emphasize.

A simple pronunciation guide:

• Ništa ≈ NEESH-ta
• ne razumijem ≈ neh rah-zoo-MEE-yem
• kad ≈ kahd
• govoriš ≈ go-VO-reesh
• prebrzo ≈ preh-BR-zoh

A few helpful notes:

• š sounds like sh
• ž sounds like the s in measure
• j is pronounced like English y
• Croatian spelling is quite regular, so pronunciation is usually predictable from the written form

It is neutral and very natural.

Because it uses govoriš, it is addressing one person informally: you singular, like ti.

If you wanted to speak formally to one person, or to more than one person, you would use govorite instead:

• Ništa ne razumijem kad govorite prebrzo.

So the original sentence is appropriate for a friend, classmate, family member, or someone you address with ti.