Ovaj veliki kofer ne stane u prtljažnik.

Because ovaj and veliki both have to agree with kofer.

In Croatian, demonstratives and adjectives match the noun in:

• gender
• number
• case

Here, kofer is:

• masculine
• singular
• nominative

So you get:

• ovaj = this for a masculine singular nominative noun
• veliki = big in the same form

Compare:

• ova velika torba = this big bag
• ovo veliko vozilo = this big vehicle

A very common clue is the ending.

Most Croatian nouns that end in a consonant are masculine, and kofer ends in -r.

That is why it takes masculine forms:

• ovaj
• veliki

This is only a rule of thumb, but it works very often.

Croatian does not have articles like a and the.

So instead of using an article, Croatian usually relies on:

• context
• word order
• demonstratives such as ovaj, taj, onaj

In this sentence, ovaj already makes the noun specific, so the idea of this big suitcase is very clear.

Here stane means fits, can go into, or can be accommodated in.

It comes from the verb stati.

In different contexts, stati can have several meanings, such as:

• to stop
• to stand up
• to fit

In this sentence, the meaning is clearly fit:

• Kofer stane u auto. = The suitcase fits in the car.
• Kofer ne stane u auto. = The suitcase does not fit in the car.

Because ne is the normal negative particle used before verbs, and it is usually written separately.

So:

• stane = fits
• ne stane = does not fit

This is the regular pattern with most verbs:

• radi → ne radi
• vidim → ne vidim
• stane → ne stane

Croatian uses the present form stane very naturally in sentences about whether something can go in somewhere.

So ne stane can correspond to English:

• doesn't fit
• won't fit

English and Croatian do not always line up word-for-word here.

A useful way to understand it is:

• Croatian is stating the result or possibility of putting it in
• English often uses fit as a general present verb

So Ovaj veliki kofer ne stane u prtljažnik is a normal Croatian way to say that the suitcase cannot fit in the trunk.

Because after u, Croatian uses different cases depending on the meaning:

• u + accusative = movement into something
• u + locative = location inside something

Here the idea is into the trunk, so you get u prtljažnik.

Compare:

• Kofer ne stane u prtljažnik. = The suitcase does not fit into the trunk.
• Kofer je u prtljažniku. = The suitcase is in the trunk.

That difference is very important in Croatian.

It is in the accusative singular.

The noun prtljažnik is masculine and inanimate. For many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: prtljažnik
• accusative: prtljažnik

Even though the form looks the same, the function is different.

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

The original sentence is a very neutral, natural order:

• Ovaj veliki kofer ne stane u prtljažnik.

But other orders are possible if you want to emphasize something:

• U prtljažnik ne stane ovaj veliki kofer.
  Emphasizes the trunk.

• Ovaj kofer ne stane u prtljažnik.
  Slightly less emphasis on big.

Even when the order changes, ne still stays with the verb: ne stane.

Yes. Gepek is a very common informal word for trunk or boot.

So you can say:

• Ovaj veliki kofer ne stane u gepek.

The difference is mainly register:

• prtljažnik = more standard
• gepek = more colloquial, everyday speech

Both are widely understood.

Usually, in a sentence like this, yes, it most naturally means the car trunk or boot.

More generally, it can refer to a luggage compartment, but in everyday use most learners will encounter it most often as car trunk.

So in British English, the sentence would usually be:

• This big suitcase doesn't fit in the boot.

And in American English:

• This big suitcase doesn't fit in the trunk.