Imam samo jedan ključ.

The I is already built into imam.
Imam is the 1st person singular present form of imati (to have), so it already means I have.

Serbian often leaves subject pronouns out when the verb makes the subject clear. You can still say Ja imam samo jedan ključ, but ja adds emphasis, as if you were saying I have only one key.

Because jedan has to agree with the noun it describes.

ključ is:

• masculine
• singular

So the correct form is jedan.

Compare:

• jedan ključ = one key
• jedna knjiga = one book
• jedno pismo = one letter

ključ is the direct object of imam, so it is in the accusative case.

However, for masculine inanimate nouns in Serbian, the accusative singular is the same as the nominative singular. That is why ključ stays ključ.

So:

• nominative: ključ
• accusative: ključ

Compare that with a masculine animate noun:

• pas = dog
• Imam psa = I have a dog

There, the accusative does change.

Because it follows jedan (one), and after one, Serbian uses a singular noun.

So:

• jedan ključ = one key

With other numbers, the noun form changes:

• dva ključa = two keys
• pet ključeva = five keys

This is something learners need to get used to, because Serbian number patterns are more grammaticalized than in English.

Serbian does not have articles like English a/an and the.

That means a sentence like Imam ključ can mean:

• I have a key
• I have the key

The exact meaning depends on context.

jedan can sometimes feel a bit like a/an, but in samo jedan ključ, it clearly keeps its numeric meaning: only one key.

samo means only or just.

In this sentence, it limits the number:

• samo jedan ključ = only one key

It tells us the speaker does not have two or more keys.

Without samo:

• Imam jedan ključ = I have one key / a key

With samo:

• Imam samo jedan ključ = I have only one key

Yes, but the meaning changes slightly.

• Imam ključ = I have a key / the key
• Imam jedan ključ = I have one key / a key
• Imam samo jedan ključ = I have only one key

So samo jedan adds emphasis to the fact that the quantity is limited to one.

No, Serbian word order is fairly flexible, but different orders sound more or less natural depending on emphasis.

The most neutral version here is:

• Imam samo jedan ključ.

Other versions are possible:

• Ja imam samo jedan ključ. = emphasis on I
• Samo jedan ključ imam. = emphasis on only one key

So the basic meaning can stay the same, but the focus shifts depending on word order.

A rough English approximation is klyooch.

Here is the breakdown:

• klj = a k plus a soft lj sound
• u = like oo in food
• č = like ch in church

The tricky part is lj. It is a soft consonant, somewhat similar to the lli sound in million, though not exactly the same.

So:

• ključ ≈ klyooch

It is one syllable.