U sobi je tamno.

Because u here means in, showing a location, and after u in that meaning Croatian uses the locative case.

• dictionary form: soba = room
• locative singular: sobi = in the room

So:

• u sobi = in the room

Compare:

• soba = a room / the room
• u sobi = in the room

A very common pattern is:

• u kući = in the house
• u školi = at school / in the school
• u sobi = in the room

It is the locative singular of soba.

Soba is a feminine noun, and its singular locative form is sobi.

A learner-friendly mini-pattern:

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

In this sentence, u sobi is a fixed kind of location phrase: u + locative.

Because Croatian often uses impersonal sentences for weather, conditions, atmosphere, and general states.

Here, je is the 3rd person singular of biti (to be), and Croatian does not need a dummy subject like English it.

English says:

• It is dark.

Croatian simply says:

• Tamno je.

So in U sobi je tamno, the verb is singular, but there is no word corresponding to English it.

This is very common:

• Vruće je. = It is hot.
• Hladno je. = It is cold.
• Kasno je. = It is late.
• Tiho je. = It is quiet.

In this sentence, tamno is best understood as a predicative word describing a condition/state, in the same type of structure as:

• hladno je
• tiho je
• mokro je

For learners, the important point is this:

• it does not agree with sobi
• it is not directly modifying room as a feminine noun

That is why you do not get tamna here.

So:

• U sobi je tamno. = It is dark in the room.
  → describing the atmosphere/condition in the room

But:

• Soba je tamna. = The room is dark.
  → describing the room itself, so the adjective agrees with soba

They are very close in meaning, but the grammar and focus are different.

U sobi je tamno

• literally: In the room, it is dark
• describes the condition inside the room
• uses an impersonal construction

Soba je tamna

• literally: The room is dark
• describes the room as the subject
• uses an adjective that agrees with soba:
  • soba = feminine singular
  • tamna = feminine singular

A simple way to remember it:

• U sobi je tamno = focus on the environment
• Soba je tamna = focus on the room as a thing

Because tamna would have to agree with a feminine noun subject, and there is no such subject here.

In U sobi je tamno, the structure is impersonal:

• u sobi = location
• je = is
• tamno = dark / it is dark

If you want tamna, you need a feminine singular noun being described, for example:

• Soba je tamna.

That works because:

• soba = feminine singular
• tamna = feminine singular adjective

So:

• U sobi je tamno. ✅
• Soba je tamna. ✅
• U sobi je tamna. ❌ in this intended meaning

No, Croatian word order is fairly flexible, though some orders sound more neutral than others.

The most neutral version here is:

• U sobi je tamno.

But you may also hear:

• Tamno je u sobi.

That usually puts more emphasis on tamno.

Croatian often moves elements around for focus, emphasis, or style. The meaning stays basically the same, but the information structure changes.

For a learner, the safest default is:

• U sobi je tamno.

Yes. U can take either the locative or the accusative, depending on meaning.

• u + locative = location
  
  • u sobi = in the room
• u + accusative = movement into
  
  • u sobu = into the room

Compare:

• On je u sobi. = He is in the room.
• On ide u sobu. = He is going into the room.

So in your sentence, there is no movement, only location, which is why it is u sobi.

In standard Croatian, no. You need je here.

So the correct sentence is:

• U sobi je tamno.

Croatian sometimes omits the verb to be in very informal styles, notes, poetry, or headlines, but that is not the normal standard pattern learners should copy.

For normal speech and writing, keep je.

The most standard way is:

• Je li u sobi tamno? = Is it dark in the room?

You may also hear the spoken/question-particle form:

• Jel' u sobi tamno?

But for learners, je li is the safer standard choice.

A short answer could be:

• Da, u sobi je tamno. = Yes, it’s dark in the room.
• Ne, u sobi nije tamno. = No, it isn’t dark in the room.

Yes, very many. This is a very productive Croatian pattern for describing conditions.

Examples:

• U sobi je hladno. = It is cold in the room.
• U sobi je toplo. = It is warm in the room.
• U sobi je tiho. = It is quiet in the room.
• U sobi je bučno. = It is noisy in the room.
• U sobi je zagušljivo. = It is stuffy in the room.

So once you understand U sobi je tamno, you can reuse the same structure easily:

• u + location
• je
• word for the condition/state