Ključ je nestao, pa ne mogu ući u stan.

Je nestao is the Croatian past tense.

Croatian usually forms the past with:

• a present-tense form of biti (to be)
• plus the past participle

So here:

• je = is/has as an auxiliary, specifically he/she/it is used for past tense formation
• nestao = disappeared / gone missing

Together, ključ je nestao means the key disappeared / the key is gone.

Why nestao and not some other form? Because ključ is:

• masculine
• singular

So the participle agrees with it:

• masculine singular: nestao
• feminine singular: nestala
• neuter singular: nestalo

For example:

• Knjiga je nestala. = The book disappeared.
• Pismo je nestalo. = The letter disappeared.

Nestao literally means disappeared / vanished / is gone. It does not specifically say who lost it.

So Ključ je nestao feels like:

• The key is gone
• The key has disappeared

If you wanted to say I lost the key, you would more naturally say:

• Izgubio sam ključ.

That sentence focuses on who caused the loss.
But Ključ je nestao focuses on the resulting situation: the key is missing.

Here pa means something like:

• so
• and so
• therefore
• sometimes even then

In this sentence, it connects cause and consequence:

• Ključ je nestao, pa ne mogu ući u stan.
• The key is gone, so I can’t enter the apartment.

It is very common in everyday Croatian and often sounds a bit more natural and conversational than a heavier connector like stoga or zato.

Because pa is linking two clauses here:

• Ključ je nestao
• ne mogu ući u stan

In Croatian, when pa connects clauses like this, a comma is normally used.

So the punctuation is standard:

• Ključ je nestao, pa ne mogu ući u stan.

This is about aspect.

• ući = perfective = to enter / to get in as a single completed action
• ulaziti = imperfective = to be entering / to enter habitually / repeatedly

Here the meaning is:

• I can’t get in
• I’m unable to enter

That is a single completed action, so Croatian uses the perfective verb ući.

If you said ne mogu ulaziti, it would sound more like:

• I’m not able to be going in / to enter repeatedly which does not fit the situation well.

After modal verbs like moći (can / be able to), Croatian normally prefers the infinitive:

• mogu ući = I can enter
• ne mogu ući = I can’t enter

The structure da + present tense is much more associated with Serbian and is generally not standard Croatian in this kind of sentence.

So for Croatian, ne mogu ući is the natural choice.

Because Croatian uses different cases depending on whether you mean:

• motion toward/into somewhere
• location inside somewhere

Here, the speaker wants to enter into the apartment, so Croatian uses:

• u + accusative

That gives:

• u stan = into the apartment

Compare:

• u stan = into the apartment
• u stanu = in the apartment

So:

• Ne mogu ući u stan. = I can’t get into the apartment.
• Ne mogu biti u stanu. = I can’t be in the apartment.

It is the accusative singular.

The noun stan is masculine and inanimate. For masculine inanimate nouns, the accusative singular is often the same as the nominative singular.

So:

• nominative: stan
• accusative: stan

Even though the form looks the same, the function is different. In u stan, it is accusative because it follows u with the meaning of movement into somewhere.

Croatian has no articles, so there is no direct equivalent of English a/an/the.

That means stan can mean:

• an apartment
• the apartment

The exact meaning depends on context.

Also, Croatian often leaves out possessives like my when they are obvious from the situation. In English, you might say:

• I can’t get into my apartment

In Croatian, it is very natural just to say:

• Ne mogu ući u stan

because the listener usually understands which apartment is meant.

If you want to be explicit, you could say:

• Ne mogu ući u svoj stan. = I can’t enter my apartment.

Ući is normally intransitive. It means to enter / to go in, and it does not take a direct object the way English enter sometimes does.

So Croatian usually expresses the destination with a prepositional phrase:

• ući u stan = enter the apartment / go into the apartment
• ući u kuću = enter the house
• ući u sobu = enter the room

That is why u stan is there: it shows where the person wants to enter.

Yes. Croatian word order is fairly flexible, although some versions sound more natural than others depending on emphasis.

The given sentence:

• Ključ je nestao, pa ne mogu ući u stan.

is very natural and neutral.

You could also say:

• Ne mogu ući u stan, jer je ključ nestao.
• Pa ne mogu ući u stan — ključ je nestao.

But these versions shift the focus a little.

The original order first states the problem:

• The key is gone and then the consequence:
• so I can’t get into the apartment

That makes it a very natural everyday sentence.

Those are different case forms, so you cannot just swap them in here.

In this sentence:

• ključ is the subject, so it must be in the nominative
• stan comes after u with movement, so it must be in the accusative

So:

• Ključ je nestao is correct
• ući u stan is correct

Forms like ključa and stana are often genitive and would appear in other contexts, for example:

• Nema ključa. = There is no key.
• Vrata stana = the apartment’s door / the door of the apartment

So the forms in the sentence are exactly the ones required by the grammar.