Ništa ne traje zauvijek.

Croatian normally uses negative concord, which means multiple negative elements appear together in the same sentence.

So:

• Ništa ne traje zauvijek. = Nothing lasts forever.

Here:

• ništa = nothing
• ne = the negative particle used with the verb
• traje = lasts

In standard Croatian, if you use a negative word like ništa, nitko, nikad, etc., the verb is also usually negated with ne.

Compare:

• Ništa ne vidim. = I see nothing.
• Nitko ne dolazi. = Nobody is coming.
• Nikad ne kasnim. = I am never late.

So this is completely normal Croatian grammar, not an error.

In this sentence, ništa is a negative pronoun meaning nothing.

It is in the nominative case because it is the subject of the sentence:

• Ništa ne traje zauvijek.
• Nothing lasts forever.

A useful thing to know is that ništa can appear in other cases too, depending on its role in the sentence. For example:

• Ne vidim ništa. = I do not see anything / I see nothing.
  Here ništa is an object.

But in your sentence, it is the subject, so you can think of it simply as nothing.

Traje is the 3rd person singular present tense form of the verb trajati, which means to last.

Why 3rd person singular?

Because the subject ništa is grammatically singular:

• ništa = nothing
• therefore the verb is singular: traje

A quick mini-conjugation of trajati in the present tense:

• ja trajem = I last
• ti traješ = you last
• on / ona / ono traje = he / she / it lasts
• mi trajemo = we last
• vi trajete = you plural/formal last
• oni / one / ona traju = they last

So Ništa ne traje zauvijek uses the correct form for a singular subject.

Zauvijek means forever.

It is an adverb, so it modifies the verb traje:

• traje zauvijek = lasts forever

It does not change for gender, number, or case.

You may also see similar words such as:

• uvijek = always
• dovijeka = forever, for all time
• vječno = eternally

But zauvijek is a very common and natural way to say forever.

Yes, Croatian word order is more flexible than English word order, so you can move parts of the sentence for emphasis.

The most neutral version is:

• Ništa ne traje zauvijek.

You could also say:

• Zauvijek ništa ne traje.

This still makes sense, but it sounds more marked or poetic, with extra emphasis on forever.

In everyday speech, the original order is the most natural and common.

In Croatian, the negative particle ne is usually written as a separate word before the verb:

• ne traje
• ne znam
• ne mogu

So in your sentence:

• Ništa ne traje zauvijek.

That is the standard spelling.

There are a few common exceptions in Croatian where negation is written together with certain forms, but with ordinary present-tense verbs like traje, you write ne separately.

Yes, Ništa ne traje vječno is also possible.

Both mean roughly:

• Nothing lasts forever.

The difference is mainly stylistic:

• zauvijek = forever
• vječno = eternally / forever

Zauvijek is very common and straightforward.
Vječno can sound a bit more literary, philosophical, or emotional in some contexts.

So:

• Ništa ne traje zauvijek. = very natural, common
• Ništa ne traje vječno. = also correct, slightly different tone

Croatian does not need a separate subject like English sometimes does, and here the subject is already expressed by ništa.

English says:

• Nothing lasts forever.

Croatian says:

• Ništa ne traje zauvijek.

So ništa already contains the idea of nothing, and that is enough to function as the subject. There is no need for another pronoun.

Also, Croatian does not use articles like a, an, or the, so the sentence is naturally shorter.

A simple learner-friendly pronunciation is:

• NI-sh-ta

More precisely:

• ni sounds roughly like nee
• št is like sht
• so ništa is approximately NEE-shta

The stress is usually on the first syllable: NÌšta.

And the whole sentence is approximately:

• NEE-shta ne TRA-ye za-U-vyek

That is only an approximation using English spelling, but it should help you get close.

The verb trajati is imperfective.

That makes sense because to last describes a continuing state or duration, not a single completed event.

Croatian often uses imperfective verbs for general truths, habits, and ongoing situations. This sentence is a general statement about life or reality:

• Ništa ne traje zauvijek.

So the imperfective verb is exactly what you would expect here.

There is no perfective partner that would normally replace trajati in this sentence with the same meaning.