Sestra je stavila kofere u prtljažnik, ali je još jednom provjerila jesmo li zatvorili vrata auta.

Because they agree with sestra, which is feminine singular.

In Croatian past tense, you usually have:

• an auxiliary form of biti = je
• plus the l-participle

So:

• stavila = feminine singular past participle of staviti
• provjerila = feminine singular past participle of provjeriti

Because the subject is sestra, the feminine form is required.

Compare:

• Brat je stavio = Brother put
• Sestra je stavila = Sister put

Je is the 3rd person singular present form of biti and helps form the past tense.

So:

• je stavila = put
• je provjerila = checked

It appears twice because there are two past-tense verb phrases:

1. Sestra je stavila kofere u prtljažnik
2. ali je još jednom provjerila...

Croatian often repeats the auxiliary in coordinated clauses like this.

Because u can take different cases depending on meaning:

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

Here the suitcases are being moved into the trunk, so Croatian uses:

• u prtljažnik = into the trunk

If you were talking about location, you would use:

• u prtljažniku = in the trunk

Kofere is the accusative plural of kofer.

It is used because kofere is the direct object of stavila:

• staviti što? → kofere

So the sentence is literally structured like:

• sister put the suitcases into the trunk

Auta is genitive singular of auto.

Croatian often uses the genitive after another noun to show possession or relation:

• vrata auta = the car door(s), literally the door(s) of the car

So:

• vrata = door(s)
• auta = of the car

You could also hear or read vrata automobila, which is a more formal version.

Još jednom literally means one more time.

In this sentence it means:

• again
• once more

So još jednom provjerila means she checked again / one more time.

Here jesmo li introduces an indirect yes/no question:

• provjerila jesmo li zatvorili vrata auta
  = she checked whether we had closed the car door(s)

Grammatically:

• jesmo = we are / have from biti
• li = question particle

Together, jesmo li means something like have we / did we in a question structure.

After verbs like provjeriti, pitati, vidjeti, it often translates as whether in English.

Because smo is a clitic form, and clitics cannot normally stand in that position by themselves in standard Croatian.

So in question-like structures you use the full form:

• jesam li
• jesi li
• je li
• jesmo li
• jeste li
• jesu li

That is why the sentence has:

• jesmo li zatvorili...

not standard smo li zatvorili...

You may also hear da li smo zatvorili... in everyday speech, but jesmo li is a very standard and natural choice here.

Because the verb agrees with the implied subject we.

In Croatian past tense, the l-participle agrees in gender and number with the subject.

• zatvorili = masculine plural
• zatvorile = feminine plural

When the group is mixed or unspecified, Croatian normally uses the masculine plural as the default.

So:

• jesmo li zatvorili = did we close / have we closed

If the speakers were all female, it could be:

• jesmo li zatvorile

Because Croatian usually omits subject pronouns when the verb form already shows who the subject is.

For example:

• je stavila already tells you it is she
• jesmo ... zatvorili already tells you it is we

Also, sestra is already explicitly named, so adding ona would usually be unnecessary.

This is very common in Croatian and other Slavic languages.

Because Croatian word order is more flexible, and short words like je and li follow special placement rules.

These short forms are clitics, and they tend to appear near the beginning of their clause.

So:

• ali je još jednom provjerila... sounds natural in Croatian
• jesmo li zatvorili... is the normal question pattern

A native English speaker often expects word order to work more like English, but Croatian relies much more on case endings and clitic placement.

Grammatically, vrata is a plural-only noun in Croatian.

That means it looks plural even when English might use singular door.

So vrata auta can refer to:

• the car door
• the car doors

The exact English translation depends on context.

This is similar to how some nouns in other languages are grammatically plural even when English treats them differently.

Yes. All three are perfective:

• staviti = to put, place
• provjeriti = to check
• zatvoriti = to close

Perfective verbs present the action as a completed whole.

That fits this sentence well:

• she put the suitcases in
• she checked again
• we closed the car door(s)

In English, these often translate simply with the past tense, but in Croatian the aspect is an important part of the meaning.