U sobi želimo promijeniti madrac, jer stari više nije udoban.

Because u sobi means in the room, so it uses the locative case after u to show location.

• u + locative = in/at a place
  
  • u sobi = in the room
• u + accusative usually shows movement toward something
  
  • u sobu = into the room

So in this sentence, nothing is moving into the room; it is just saying where the mattress change is being considered, so u sobi is correct.

Sobi is the locative singular of soba (room).

Here is the pattern:

• nominative: soba
• genitive: sobe
• dative: sobi
• accusative: sobu
• locative: sobi
• instrumental: sobom

After the preposition u meaning in, Croatian normally uses the locative, so:

• u sobi = in the room

Because Croatian often uses htjeti/željeti + infinitive to say want to do something.

• želimo = we want
• promijeniti = to change / to replace

So:

• želimo promijeniti madrac = we want to change/replace the mattress

This works much like English want to change.

This is about aspect, which is very important in Croatian.

• mijenjati = imperfective, focusing on the process or repeated action
• promijeniti = perfective, focusing on completing the change

In this sentence, the idea is to replace the mattress, meaning to make a complete change, so promijeniti is the natural choice.

Very roughly:

• želimo mijenjati madrac = we want to be changing the mattress / keep changing it / the process matters
• želimo promijeniti madrac = we want to replace the mattress

It is in the accusative singular, because it is the direct object of promijeniti.

The reason it looks unchanged is that madrac is a masculine inanimate noun, and in Croatian those often have:

• nominative singular = madrac
• accusative singular = madrac

So the form stays the same, even though the case has changed.

Here stari means the old one.

The full phrase would be something like:

• stari madrac = the old mattress

But Croatian often leaves out the noun when it is obvious from context. So:

• stari više nije udoban = the old one is no longer comfortable

This is very natural. The adjective stari is being used almost like a noun here.

Because it refers to madrac, which is a masculine singular noun.

So the adjective must agree with madrac in gender, number, and case.

• madrac = masculine singular
• therefore: stari and udoban

You may notice that masculine adjectives can appear in forms like:

• star = a short form, often predicate or more limited usage
• stari = the form commonly used before a noun, and also often when the noun is omitted in this kind of expression

In this sentence, stari is the natural choice for the old one.

Više nije means is no longer.

In this sentence:

• više = anymore / no longer
• nije = is not

Together:

• stari više nije udoban = the old one is no longer comfortable

Be careful: više can also mean more, but here with a negative verb it means no longer / anymore.

Examples:

• Više ne pušim. = I don't smoke anymore.
• Nije više ovdje. = He/She is no longer here.

Because nije is the normal negative form of je (is).

Croatian forms the present tense negative of biti (to be) as one word:

• nisam = I am not
• nisi = you are not
• nije = he/she/it is not
• nismo = we are not
• niste = you are not
• nisu = they are not

So:

• nije udoban = is not comfortable

You should learn these as fixed forms.

Because udoban agrees with the implied noun madrac.

Since madrac is:

• masculine
• singular
• nominative here in the second clause as the subject idea behind stari

the adjective is also masculine singular:

• udoban = comfortable

Compare:

• madrac je udoban = the mattress is comfortable
• soba je udobna = the room is comfortable
• kreveto je udobno = the bed is comfortable

Agreement is very important in Croatian adjectives.

Jer means because.

So the sentence is:

• U sobi želimo promijeniti madrac, jer stari više nije udoban.
• We want to replace the mattress in the room, because the old one is no longer comfortable.

Yes, it can often be replaced by zato što, which also means because:

• ..., jer stari više nije udoban.
• ..., zato što stari više nije udoban.

Both are common, though jer is short and very frequent in everyday language.

The word order is fairly natural, but Croatian word order is more flexible than English.

This sentence could be rearranged in some ways without changing the basic meaning, for example:

• Želimo promijeniti madrac u sobi, jer stari više nije udoban.
• Jer stari više nije udoban, želimo promijeniti madrac u sobi.

However, the original version sounds natural because it starts with the location:

• U sobi = in the room

Croatian often moves elements around for emphasis, topic, or style, but not all word orders sound equally natural in every context.

In this context, it most naturally means replace the mattress.

In English, change the mattress can sound slightly ambiguous, but in Croatian:

• promijeniti madrac usually means to swap it out for another one

So the sentence is really saying that they want a new mattress because the old one is no longer comfortable.

Because Croatian has no definite or indefinite articles like English a/an/the.

So Croatian simply says:

• u sobi = in the room / in a room
• madrac = mattress / the mattress / a mattress
• stari = the old one / old one

The exact meaning depends on context. In this sentence, the context clearly makes them definite:

• u sobi = in the room
• madrac = the mattress
• stari = the old one

That is completely normal in Croatian.