När hon såg sin syster på perrongen började hon ropa och sprang fram för att krama henne.

This sentence contains two clauses:

• När hon såg sin syster på perrongen = a subordinate clause
• började hon ropa och sprang fram för att krama henne = the main clause

In the subordinate clause, Swedish keeps the normal subject-verb order: hon såg.

But because the sentence starts with that subordinate clause, the main clause follows the V2 rule: the finite verb must come first in the main clause after the opening element. So you get:

• När hon såg sin syster på perrongen, började hon ...

That is why it is började hon, not hon började.

Sin is a reflexive possessive, and it refers back to the subject of its own clause.

In När hon såg sin syster, the subject of that clause is hon, so sin syster means her own sister.

If you said hennes syster, it would normally mean another woman's sister, not the subject's own sister.

A useful contrast:

• Hon såg sin syster = she saw her own sister
• Hon såg hennes syster = she saw someone else's sister

Normally, yes. Most readers will understand both hon forms as the same woman.

Grammatically, Swedish does not make that absolutely impossible to read another way, but in real usage this sentence strongly suggests one continuous subject: she saw her sister, then she started shouting, then she ran forward.

If two different women were meant, a native speaker would usually rewrite the sentence to make that clearer.

They are all in the preterite (simple past).

• såg = past of se
• började = past of börja
• sprang = past of springa

Swedish often uses the simple past where English also uses the simple past, so you do not need an extra auxiliary here.

After börja, Swedish often uses a bare infinitive:

• började ropa

You can also hear började att ropa, but leaving out att is very common and natural.

So började ropa is completely normal Swedish.

Here ropa means something like call out, shout, or cry out, depending on context.

It is a general verb for using your voice loudly enough to get attention. In this sentence, it suggests an excited reaction when she sees her sister.

Fram adds the idea of movement forward or up toward someone/something.

So sprang fram is not just ran in a general sense. It means she ran up to her sister.

This is very common in Swedish: a verb plus a particle gives a more specific sense of movement.

För att introduces purpose. It means in order to.

So sprang fram för att krama henne means she ran forward in order to hug her.

After för att, Swedish uses the infinitive:

• för att krama
• för att hjälpa
• för att se

Because henne is the object form.

After the verb krama, you need an object pronoun:

• hon = subject form
• henne = object form

So:

• hon kramade henne = she hugged her

You cannot use hon there, because hon cannot be the direct object.

Perrongen is the definite form, meaning the platform.

The sentence refers to a specific platform in the situation, not just any platform. Swedish often uses the definite form when the place is understood as a particular one in the scene.

So på perrongen means on the platform or at the platform, depending on how the meaning has been shown.

Swedish uses på with many places and surfaces where English also often uses on or sometimes at.

With perrong it is standard to say:

• på perrongen

So this is just the normal preposition for being on a station platform.

In Swedish, a comma after an initial subordinate clause is often optional, especially when the sentence is not very hard to read.

So both of these can occur:

• När hon såg sin syster på perrongen började hon ropa ...
• När hon såg sin syster på perrongen, började hon ropa ...

Many Swedish writers leave the comma out unless it helps clarity or rhythm. English often uses commas more readily in this kind of sentence.