Ona je gurnula vrata, ali se nisu otvorila.

Because Croatian often can drop subject pronouns, but speakers still use them for emphasis, contrast, or clarity.

So:

• Ona je gurnula vrata... = She pushed the door
• Gurnula je vrata... = also natural, with she understood from the verb form

Here, ona may suggest a bit of emphasis, like she pushed it, but it still didn’t open.

It is the normal Croatian past tense (often called perfekt).

It is formed with:

• a present-tense form of biti = je
• plus the -l participle = gurnula

So:

• ona je gurnula = she pushed

The second clause works the same way:

• nisu otvorila = did not open

There, nisu is the negative auxiliary, and otvorila is the participle.

Because the participle agrees with the subject in gender and number.

The subject is ona, which is:

• feminine
• singular

So the participle is feminine singular:

• on je gurnuo = he pushed
• ona je gurnula = she pushed
• ono je gurnulo = it pushed

So -la here shows feminine singular agreement.

Here vrata is the direct object of gurnula, so it is in the accusative case.

However, vrata is a special noun:

• it is neuter
• it is normally used only in the plural
• its nominative plural and accusative plural are both vrata

So the form does not change here.

That is why you see:

• gurnula vrata = pushed the door / doors

Because vrata is a plural-only noun for door in Croatian.

This is one of those nouns that are grammatically plural even when English often uses a singular word.

So Croatian says:

• vrata = door / doors, depending on context

This is similar in spirit to how some English nouns behave unusually, though not exactly the same way.

As a result, Croatian grammar treats vrata as plural.

Because in the second clause, vrata is the subject, and the verb agrees with it.

So the full idea is really:

• Ona je gurnula vrata, ali se vrata nisu otvorila.

The second vrata is omitted because it is obvious from context.

Since vrata is grammatically plural, the verb is plural too:

• nisu = plural auxiliary
• otvorila = neuter plural participle

Even if English translates this with singular the door, Croatian still uses plural agreement because vrata is grammatically plural.

Not here. This is an important point.

In gurnula, -la is feminine singular because it agrees with ona.

In otvorila, the same-looking ending is neuter plural because it agrees with vrata.

So these two forms look the same on the surface, but they mean different agreement patterns:

• gurnula = feminine singular
• otvorila = neuter plural

You understand which one it is from the subject and the rest of the sentence.

Here se helps make the verb intransitive or middle in meaning.

Compare:

• otvoriti vrata = to open the door
• vrata su se otvorila = the door opened

So this sentence does not mean she did not open them.
It means they did not open.

In other words:

• otvoriti = open something
• otvoriti se = open

The se does not have to be translated literally as itself.

Because se is a clitic, and Croatian clitics tend to go in the second position of the clause.

After ali, the clitic comes right away:

• ali se nisu otvorila

Also, the negative auxiliary nisu behaves differently from a normal unstressed auxiliary, so se comes before it here.

That is why:

• ali se nisu otvorila = correct
• ali nisu se otvorila = not standard

Compare the affirmative version:

• ali su se otvorila = but they opened

Yes, aspect is very important in Croatian.

Both verbs here are perfective:

• gurnuti = to give a push, push once/completely
• otvoriti se = to open, become open as a completed event

That fits the sentence well:

• she gave the door a push
• but it still did not open

If you wanted a more ongoing or repeated sense, you would usually use imperfective verbs, for example:

• gurala je vrata = she was pushing / kept pushing the door
• vrata se nisu otvarala = the door was not opening

So aspect helps show whether the action is viewed as a completed event or as ongoing/repeated.

Because Croatian often omits repeated words when they are already clear from context.

The full version could be:

• Ona je gurnula vrata, ali se vrata nisu otvorila.

But repeating vrata is unnecessary here, so it is usually left out:

• Ona je gurnula vrata, ali se nisu otvorila.

The subject of the second clause is understood to be vrata.