Ne mogu ući u zgradu bez ključa.

Yes. Ne mogu means I cannot / I can’t.

• mogu = I can / I am able to
• ne = the negative particle not

In Croatian, ne is normally written as a separate word before the verb:

• mogu = I can
• ne mogu = I can’t

So Ne mogu ući... literally means I cannot enter...

Mogu is the 1st person singular present tense of moći (to be able to / can).

A few useful forms are:

• ja mogu = I can
• ti možeš = you can
• on/ona može = he/she can
• mi možemo = we can
• vi možete = you can (plural/formal)
• oni mogu = they can

In the sentence, the subject ja (I) is omitted because Croatian usually leaves it out when it is clear from the verb form.

Ući means to enter / to go in, and it is a perfective verb. That means it presents the action as a single complete event: actually getting inside.

So Ne mogu ući u zgradu means:

• I can’t get into the building
• I can’t enter the building

By contrast, ulaziti is imperfective and is more like:

• to be entering
• to enter habitually / repeatedly
• to go in as an ongoing or repeated action

In this sentence, ući is the natural choice because the idea is one completed act of entry.

They are related in form, but they are doing different jobs.

• ući is the verb: to enter
• u before zgradu is the preposition: into / in

So:

• ući = to go in, to enter
• u zgradu = into the building

This is very common in Croatian. You can have a verb with the idea of movement inward, and then also use the preposition u with the place you are moving into.

Because Croatian uses different cases depending on whether you mean movement toward/into a place or location in a place.

With u:

• u + accusative = movement into
• u + locative = location in

So:

• u zgradu = into the building → movement, so accusative
• u zgradi = in the building → location, so locative

In your sentence, the speaker is trying to enter the building, so movement is involved. That is why it is u zgradu.

Because zgrada is a feminine noun, and here it is in the accusative singular after u expressing movement.

Its basic form is:

• zgrada = building

But in this sentence, you need the accusative:

• u zgradu = into the building

For many feminine nouns ending in -a, the accusative singular ends in -u:

• kuća → kuću
• škola → školu
• zgrada → zgradu

Because the preposition bez (without) requires the genitive case.

So:

• ključ = key
• bez ključa = without a key / without the key

This is something you simply have to learn with the preposition:

• bez + genitive

More examples:

• bez vode = without water
• bez novca = without money
• bez automobila = without a car

Ključa is the genitive singular of ključ.

• ključ = key
• ključa = of a key / of the key

In this sentence, it appears because bez requires the genitive:

• bez ključa = without a key

So the noun itself is masculine:

• nominative singular: ključ
• genitive singular: ključa

It can mean either, depending on context.

Croatian has no articles like a and the, so bez ključa can be understood as:

• without a key
• without the key

In most everyday contexts, English would naturally translate it as without a key unless the situation makes a specific key obvious.

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

That means nouns often appear without any separate word showing definiteness. Whether something means a building or the building is usually understood from context.

So u zgradu could be translated as:

• into a building
• into the building

In your full sentence, the building is the most natural English translation if the building is already known from context.

A rough guide:

• ući sounds approximately like OO-chee, but the ć is a softer sound than English ch
• ključa sounds roughly like KLYOO-cha, with lj pronounced like a soft ly sound together

A few details:

• ć is a soft consonant, somewhat between English t and ch
• lj is treated as one sound in Croatian, similar to the lli in some pronunciations of million, though not exactly the same
• ž in zgrada is like the s in measure

Approximate whole-sentence pronunciation:

Ne mogu ući u zgradu bez ključa.
≈ Neh MOH-goo OO-chee oo ZGRAH-doo bez KLYOO-cha.

The given word order is the most neutral and natural:

Ne mogu ući u zgradu bez ključa.

Croatian word order is more flexible than English, but changes usually affect emphasis. For example, you might move something earlier to stress it, but the neutral version is best for learners.

So yes, word order can sometimes vary, but this sentence is the standard way to say it.

By itself, Ne mogu most directly means I can’t / I’m unable to. But in real life, that can overlap with I’m not allowed to, depending on context.

For example:

• if the door is locked, it means I’m unable to
• if the rules require a key card, it may feel like I’m not allowed to

Still, grammatically, ne mogu is about lack of ability/possibility more than explicit prohibition. If you want to clearly say I’m not allowed to, Croatian often uses expressions like ne smijem.

• Ne smijem ući u zgradu bez ključa. = I’m not allowed to enter the building without a key.