U prtljažniku je rezervni kotač, za svaki slučaj.

Because u changes case depending on meaning:

• u + locative = location, meaning in / inside
• u + accusative = motion into something

Here the sentence describes where the spare wheel is, so prtljažnik becomes prtljažniku in the locative singular:

• u prtljažniku = in the trunk / boot

Compare:

• Rezervni kotač je u prtljažniku. = The spare wheel is in the trunk.
• Stavio sam rezervni kotač u prtljažnik. = I put the spare wheel into the trunk.

It is locative singular.

The dictionary form is prtljažnik. After u when it means location, masculine nouns usually go into the locative:

• prtljažnik → u prtljažniku

This is a very common pattern in Croatian:

• u autu = in the car
• u gradu = in the city
• u hotelu = in the hotel

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

This sentence begins with the location:

• U prtljažniku je rezervni kotač.

That feels natural because it first sets the scene: in the trunk...

Also, je is a clitic. Croatian clitics usually come very early in the sentence, often in the second position. So after the opening phrase u prtljažniku, the clitic je comes next.

You could also say:

• Rezervni kotač je u prtljažniku.

That is also correct, but it gives a slightly different emphasis.

Croatian does not need a separate word like English there in this kind of sentence.

So English:

• There is a spare wheel in the trunk.

becomes Croatian:

• U prtljažniku je rezervni kotač.

Literally, it is closer to:

• In the trunk is a spare wheel.

That is a normal way to express existence or presence in Croatian.

Because the adjective must agree with the noun in:

• gender
• number
• case

Here kotač is a masculine singular noun in the nominative, so the adjective must also be masculine singular nominative:

• rezervni kotač

Compare:

• rezervna guma = spare tire
  because guma is feminine
• rezervno kolo = spare wheel
  because kolo is neuter

Not exactly.

• kotač = wheel
• guma = tire

In everyday speech, people often say rezervna guma even when they mean the spare wheel/tire kept in the car. So both may come up, depending on context and speaking style.

In your sentence, rezervni kotač is perfectly normal and clear.

Also, depending on region, you may hear točak instead of kotač, but kotač is standard Croatian.

It is a very common fixed expression meaning:

• just in case
• for any case / eventuality

Literally:

• za = for
• svaki = every / any
• slučaj = case

So the whole phrase is an idiom: za svaki slučaj = just in case.

You can use it in many situations:

• Ponesi kišobran za svaki slučaj. = Take an umbrella just in case.
• Nazvat ću ga za svaki slučaj. = I’ll call him just in case.

Because za here takes the accusative case.

The noun slučaj is masculine singular, and in the accusative singular for this type of noun it looks the same as the nominative:

• slučaj → slučaj

The adjective must agree with it:

• svaki slučaj

So:

• za svaki slučaj = for any case / just in case

Here za svaki slučaj works like an added comment: just in case.

The comma helps show that it is slightly parenthetical, almost like an afterthought:

• U prtljažniku je rezervni kotač, za svaki slučaj.

Without the comma, the meaning would still be understandable:

• U prtljažniku je rezervni kotač za svaki slučaj.

But that version can sound a bit more tightly connected, as if the wheel is specifically intended for emergencies. With the comma, it more clearly sounds like the speaker is adding just in case as a side comment.

Because Croatian has no articles.

English distinguishes:

• a spare wheel
• the spare wheel

Croatian usually leaves that to context, word order, or emphasis:

• rezervni kotač can mean a spare wheel or the spare wheel, depending on context.

In this sentence, English naturally translates it as a spare wheel, but Croatian itself does not mark that with an article.

A few helpful points:

• prtljažniku
  The tricky part is prtlj-. Croatian allows consonant clusters that may feel hard for English speakers. Try breaking it up mentally: prt-ljaž-ni-ku.

• kotač
  The č sounds like ch in church, but a bit firmer: ko-tach.

• slučaj
  č again is like ch, and the final aj sounds like eye: roughly sloo-chy or more accurately sloo-chai.

Also remember:

• ž sounds like the s in measure
• j sounds like English y

So prtljažnik has that zh sound in the middle.

Yes, absolutely.

That version is grammatically correct too. The difference is mainly emphasis:

• U prtljažniku je rezervni kotač...
  starts with the location, so it answers Where is it?
• Rezervni kotač je u prtljažniku...
  starts with the thing itself, so it focuses more on What is there?

Both are natural. Croatian often changes word order to shift emphasis rather than basic meaning.