Ključ je ispod stola u kuhinji.

Because ispod requires the genitive case, so stol changes to stola.

• dictionary form: stol = table
• after ispod = under/beneath, you use genitive
• so: ispod stola = under the table

This is very common in Croatian: certain prepositions force a specific case.

Because u here means in, describing location, and with that meaning it usually takes the locative case.

• dictionary form: kuhinja = kitchen
• locative singular: kuhinji
• so: u kuhinji = in the kitchen

Compare:

• u kuhinji = in the kitchen (location)
• u kuhinju = into the kitchen (movement, accusative)

Je is the 3rd person singular form of the verb biti (to be).

So:

• ključ = key
• je = is

Together: Ključ je... = The key is...

In present-tense Croatian, forms of biti are often short clitic forms like sam, si, je, smo, ste, su.

Usually, no in a normal neutral sentence like this.

Croatian sometimes omits to be in certain styles, headlines, or very informal contexts, but in a standard full sentence you would normally say:

• Ključ je ispod stola u kuhinji.

Leaving out je would sound incomplete to a learner’s ear and is not the standard choice here.

Croatian does not have articles like English a/an/the.

So ključ can mean:

• a key
• the key

and stola can mean:

• of a table
• of the table

The exact meaning comes from context. In this sentence, English usually translates it as The key is under the table in the kitchen, but Croatian does not mark the explicitly.

Not completely. Croatian word order is fairly flexible because the cases show the grammatical relationships.

The neutral order here is:

• Ključ je ispod stola u kuhinji.

But you may also hear:

• Ispod stola u kuhinji je ključ.
• U kuhinji je ključ ispod stola.

These can sound more marked, depending on what the speaker wants to emphasize.

One important point: je is a clitic, so it tends to appear in the second position of its clause.

Because short forms like je often behave as clitics in Croatian. Clitics usually come near the beginning of the clause, often in second position.

So:

• Ključ je ispod stola u kuhinji.

is natural because je comes right after the first element, Ključ.

This is a common pattern in Croatian and other South Slavic languages.

Ispod takes the genitive.

Examples:

• ispod stola = under the table
• ispod kreveta = under the bed
• ispod mosta = under the bridge

So in your sentence, stola is genitive because it follows ispod.

Here u takes the locative because it expresses location: in the kitchen.

A very useful rule is:

• u + locative = in/at somewhere
• u + accusative = into somewhere

So:

• u kuhinji = in the kitchen
• u kuhinju = into the kitchen

The noun ključ is masculine.

You often see this from how adjectives or past-tense forms agree with it in other sentences. For example:

• Mali ključ = small key
  (mali is masculine singular)

Also, many Croatian masculine nouns end in a consonant, and ključ does too. That is a very common pattern, though not an absolute rule.

Yes, strictly speaking, Croatian leaves that open because there are no articles.

Without context, Ključ je ispod stola u kuhinji could correspond to several English versions:

• The key is under the table in the kitchen.
• A key is under a table in a kitchen.
• The key is under a table in the kitchen.

In real use, context usually makes the intended meaning clear.

It most naturally tells you where the whole situation is: the key is under the table, and this is happening in the kitchen.

So the sentence is understood as:

• The key is under the table, in the kitchen.

In practice, that usually also implies the table is in the kitchen. Croatian allows this kind of stacking of location phrases quite naturally.

Yes, but it is not always exactly the same in feel.

• ispod stola = under/beneath the table
• pod stolom = under the table

Both can work in many situations. For a learner, ispod + genitive is very straightforward and clearly means beneath/under.
Pod is also very common, but it uses different cases depending on meaning:

• pod stolom = location (instrumental)
• pod stol = motion toward a position under the table (accusative)

So the sentence with ispod stola is a good clear model.

A rough pronunciation for an English speaker is:

• klyooch yees-pod STOH-lah oo KOO-khee-nyee

A few helpful notes:

• ključ: č sounds like ch in church
• lj is a soft sound, somewhat like the lli in some pronunciations of million, though not exactly
• j is pronounced like English y
• h is pronounced, unlike in many English words

If you want a rougher word-by-word guide:

• Ključ ≈ klyooch
• je ≈ yeh
• ispod ≈ ees-pod
• stola ≈ stoh-la
• u ≈ oo
• kuhinji ≈ koo-khin-yee

Here they are:

• ključ → dictionary form already: ključ (key)
• je → from biti (to be)
• ispod → preposition meaning under/beneath
• stola → dictionary form: stol (table)
• u → preposition meaning in/into
• kuhinji → dictionary form: kuhinja (kitchen)

This is a useful habit in Croatian: when you see a changed noun form like stola or kuhinji, try to identify its base form first.