Jediný radiátor je v pokoji.

Jediný means only, single, or sole.

So jediný radiátor means the only radiator / a single radiator, depending on context. In this sentence, the idea is that there is just one radiator.

Grammatically, jediný is an adjective, so it agrees with radiátor in gender, number, and case.

Because it is agreeing with radiátor, which is:

• masculine inanimate
• singular
• nominative

The nominative singular ending for a hard adjective in masculine inanimate is -ý, so you get:

• jediný radiátor

If the noun changed, the adjective would change too.

Radiátor is the subject of the sentence, so it is in the nominative case.

The basic structure is:

• Jediný radiátor = the subject
• je = is
• v pokoji = in the room

So radiátor stays in its dictionary form here.

Because the preposition v meaning in normally takes the locative case when it expresses location.

So:

• pokoj = room
• v pokoji = in the room

This is a very common pattern in Czech:

• v domě = in the house
• ve škole = in the school
• v autě = in the car

Here, the sentence is talking about where something is located, so the locative is required.

It is the locative singular of pokoj.

With masculine inanimate nouns like pokoj, the locative singular often ends in -u or -i/-ě, depending on the word. For pokoj, the correct form after v is v pokoji.

In a normal sentence like this, je is required.

Czech often drops subject pronouns because the verb ending already shows the person, but it does not normally drop the verb to be in standard sentences like this.

So:

• Jediný radiátor v pokoji is not a complete normal sentence by itself
• Jediný radiátor je v pokoji is a complete sentence

Because Czech has no articles.

English uses a and the, but Czech usually leaves that idea to context.

So radiátor can mean:

• a radiator
• the radiator

and v pokoji can mean:

• in a room
• in the room

In this sentence, jediný already strongly suggests a definite meaning, so English often translates it as the only radiator.

Yes. Czech word order is fairly flexible, and changing it usually changes the focus or emphasis, not the basic meaning.

For example:

• Jediný radiátor je v pokoji.
  Focuses first on the only radiator.

• V pokoji je jediný radiátor.
  A very natural alternative; it presents the location first: In the room, there is only one radiator.

Both are grammatical. The difference is mainly in what information is being highlighted.

It is grammatical, but many speakers would very naturally also say:

• V pokoji je jen jeden radiátor.

That version may feel more directly like There is only one radiator in the room.

The original sentence, Jediný radiátor je v pokoji, can sound a bit more like The only radiator is in the room, where jediný radiátor is treated as one unit at the start.

So the best version depends on what you want to emphasize.

Czech stress is almost always on the first syllable of the word.

So radiátor is stressed like this:

• RA-di-á-tor

Also, á is a long vowel, so it is pronounced longer than a short a.

And in pokoji, the j is pronounced like English y, so it sounds roughly like:

• po-ko-yi

Not exact English pronunciation, but that gives the general idea.