Ključ je u ključanici.

Because after u meaning in, Croatian normally uses the locative case when you are talking about a location.

• ključanica = nominative singular
• u ključanici = locative singular, meaning in the keyhole / in the lock

So:

• Ključanica je... = The keyhole / lock is...
• u ključanici = in the keyhole / lock

For many feminine nouns ending in -a, the locative singular ends in -i:

• soba → u sobi
• škola → u školi
• ključanica → u ključanici

Because ključ is the subject of the sentence, so it stays in the nominative case.

In Ključ je u ključanici, the thing being talked about is the key, so Croatian uses the basic dictionary form:

• ključ = key

You can think of the structure as:

• Ključ = subject
• je = is
• u ključanici = location phrase

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

So:

• ja sam = I am
• ti si = you are
• on / ona / ono je = he / she / it is

Since ključ is singular, Croatian uses je:

• Ključ je... = The key is...

Croatian does not have articles like English a and the.

So ključ can mean:

• a key
• the key

and ključanica can mean:

• a keyhole / a lock
• the keyhole / the lock

The exact meaning depends on context. In this sentence, English usually translates it as The key is in the keyhole / lock, but Croatian does not need a separate word for the.

U usually means in or inside.

Here it shows where the key is located:

• u ključanici = in the keyhole / lock

Croatian often uses:

• u + locative for being in a place
• u + accusative for motion into a place

Compare:

• Ključ je u ključanici. = The key is in the keyhole.
• Stavio sam ključ u ključanicu. = I put the key into the keyhole.

That is a very important pattern in Croatian.

The difference is location vs motion.

• u ključanici = in the keyhole / lock → location, no movement
• u ključanicu = into the keyhole / lock → direction or movement

Examples:

• Ključ je u ključanici. = The key is in the keyhole.
• Stavi ključ u ključanicu. = Put the key into the keyhole.

This is one of the most common case patterns after prepositions in Croatian.

A rough English-style guide is:

klyooch yeh oo klyoo-cha-nee-tsee

A few important pronunciation points:

• lj in ključ is a single sound, not a fully separate l plus j
• č sounds like ch in church
• j sounds like English y in yes
• c sounds like ts
• Croatian stress is not usually marked in normal writing, so learners often just imitate native audio at first

So:

• ključ ≈ klyooch
• je ≈ yeh
• ključanici ≈ klyoo-cha-nee-tsee

In everyday usage, ključanica is often understood as keyhole, but depending on context it can be associated with the lock area where the key goes in.

Learners should usually remember it as:

• ključ = key
• ključanica = keyhole

If a speaker wants to refer more generally to the locking mechanism, other words may appear depending on context, such as brava for lock.

So in this sentence, the safest basic understanding is:

• Ključ je u ključanici. = The key is in the keyhole.

Because ključanica is related to ključ.

• ključ = key
• ključanica = keyhole / lock part for the key

This kind of word family is common in Croatian and can help vocabulary learning. If you recognize the root ključ-, it becomes easier to guess the meaning of related words.

Yes. Croatian word order is more flexible than English word order.

The neutral order here is:

• Ključ je u ključanici.

But you could also hear:

• U ključanici je ključ.

Both mean the same basic thing, but the emphasis changes.

• Ključ je u ključanici. focuses first on the key
• U ključanici je ključ. focuses first on where it is

Croatian often moves parts of the sentence around for emphasis, topic, or style.

Croatian has a strong tendency for short unstressed words, called clitics, to appear near the second position in a clause. Je is one of them.

So in:

• Ključ je u ključanici.

the clitic je comes right after the first phrase Ključ.

This is very normal in Croatian. You will see the same thing with other short forms such as:

• sam
• si
• je
• ga
• mi
• se

This word-order rule is important and comes up all the time.

Yes. Ključ is a masculine noun.

A useful rule of thumb is:

• nouns ending in a consonant are often masculine
• nouns ending in -a are often feminine
• nouns ending in -o or -e are often neuter

So here:

• ključ ends in a consonant → masculine
• ključanica ends in -a → feminine

That helps explain why ključanica changes to ključanici in the locative.

Normally, if you want a full sentence meaning The key is in the keyhole, you should keep je:

• Ključ je u ključanici.

If you say only:

• Ključ u ključanici

that sounds more like a note, label, headline, or fragment, not a complete neutral sentence.

Croatian sometimes drops forms of to be in very informal or special styles, but for standard learning purposes, keep je here.