Kad sam je vidjela na kolodvoru, mahnula sam joj iz vlaka.

Those forms show that the speaker is female.

In Croatian past tense, the main verb agrees with the subject in gender and number:

• vidjela, mahnula = feminine singular
• vidio, mahnuo = masculine singular
• vidjelo, mahnulo = neuter singular

So a male speaker would say:

Kad sam je vidio na kolodvoru, mahnuo sam joj iz vlaka.

Because there are two separate past-tense clauses, and each one has its own past-tense auxiliary.

Croatian perfect tense is built with:

• the auxiliary biti in the present tense
• plus a past participle

So here you have:

• sam je vidjela = I saw her
• sam joj mahnula = I waved to her

Even though both actions are in one sentence, each clause still needs its own sam.

Here je means her, not is.

It is a short, unstressed object pronoun. In this sentence, it is the direct object of vidjela:

• vidjeti nekoga = to see someone

So:

• je = her

This can confuse learners because je can also mean is in other sentences. Here, though, it is definitely the pronoun her.

Because they are different cases, with different functions.

• je = direct object, her
• joj = indirect object, to her

In this sentence:

• vidjeti nekoga → see someone → direct object → je
• mahnuti nekome → wave to someone → indirect object → joj

So the sentence literally works like:

• When I saw her at the station, I waved to her from the train.

English uses word order and prepositions; Croatian often shows these relationships through case.

Because this is about location, not movement.

• na kolodvoru = at the station / on the station premises → location
• na kolodvor = to the station → movement toward

Here the speaker saw her while she was already at the station, so Croatian uses na + locative:

• na kolodvoru

Compare:

• Vidjela sam je na kolodvoru. = I saw her at the station.
• Idem na kolodvor. = I’m going to the station.

Because iz takes the genitive case.

The noun is:

• vlak = train

After iz, it becomes:

• iz vlaka = from the train

This phrase shows where the waving came from.

A useful contrast:

• iz vlaka = from/out of the train
• u vlaku = in the train

In this sentence, iz vlaka sounds natural because the speaker is waving from inside the train outward.

This is a question of aspect.

• mahnuti = perfective → one completed wave
• mahati = imperfective → waving repeatedly or continuously

So:

• mahnula sam joj = I gave her a wave
• mahala sam joj = I was waving to her / I waved to her repeatedly

In this sentence, the idea is probably a single, completed gesture after seeing her, so mahnula fits well.

Because Croatian has a system of unstressed words called clitics, and they tend to stand near the beginning of a clause, usually in the “second position.”

The clitics here are:

• sam
• je
• joj

So:

• Kad sam je vidjela = after Kad, the clitics come early
• mahnula sam joj = the clause starts with mahnula, and then the clitics follow

This is very normal Croatian word order.

Also, clitic order inside the group is not random. Croatian has preferred patterns, so sam je and sam joj are the expected combinations.

Because Croatian usually leaves subject pronouns out when they are not needed.

The verb already tells you the person:

• sam ... vidjela
• sam ... mahnula

That clearly shows I.

So ja is often omitted unless you want emphasis or contrast:

• Ja sam joj mahnula, a on nije. = I waved to her, but he didn’t.

In the original sentence, ja would be unnecessary.

Yes. Both mean when.

• kad = very common, everyday
• kada = also correct, sometimes a bit fuller or slightly more formal

So both are fine:

• Kad sam je vidjela...
• Kada sam je vidjela...

The meaning does not change here.

Because the sentence begins with a subordinate time clause:

• Kad sam je vidjela na kolodvoru = When I saw her at the station

That clause is followed by the main clause:

• mahnula sam joj iz vlaka

In Croatian, it is normal to separate that introductory subordinate clause with a comma.

Yes. Croatian word order is flexible, and moving parts of the sentence usually changes emphasis more than basic meaning.

For example:

• Kad sam je vidjela na kolodvoru, iz vlaka sam joj mahnula.
• Mahnula sam joj iz vlaka kad sam je vidjela na kolodvoru.

These still mean roughly the same thing, but the focus shifts a little:

• iz vlaka earlier can emphasize where the wave came from
• putting the kad-clause later can make it sound less like the main frame of the sentence

The original version is a natural, neutral way to say it.