Hon längtar efter sin syster och hoppas att de kan ses i kväll.

In Swedish, längta usually takes the preposition efter when you say what or whom someone longs for.

• längta efter någon = to long for someone
• längta efter något = to long for something

So:

• Hon längtar efter sin syster = she longs for her sister

This is just the normal pattern of the verb. It is similar to how some English verbs need a fixed preposition, even when the preposition does not translate word-for-word.

Sin is a reflexive possessive pronoun. It is used when the possessor is the same person as the subject of the clause.

In this sentence:

• Hon = the subject
• sin syster = her own sister

So sin shows that the sister belongs to hon.

Compare:

• Hon längtar efter sin syster = She longs for her own sister.
• Hon längtar efter hennes syster = She longs for another woman’s sister.

This is one of the most important Swedish differences from English, because English uses her for both meanings.

These are all forms of the same reflexive possessive word:

• sin — used with common-gender singular nouns
• sitt — used with neuter singular nouns
• sina — used with plural nouns

Examples:

• sin syster = his/her own sister
• sitt hus = his/her own house
• sina böcker = his/her own books

In your sentence, syster is a common-gender singular noun, so sin is the correct form.

Here, att introduces a subordinate clause, like that in English:

• hoppas att de kan ses = hopes that they can meet / see each other

In English, that is often omitted:

• She hopes (that) they can meet tonight.

In Swedish, att is very commonly kept in this kind of sentence. It sounds natural and standard here.

Because att de kan ses i kväll is a subordinate clause.

In a normal subordinate clause introduced by att, Swedish keeps the subject before the verb:

• att de kan ses i kväll

That is the standard word order:

• att
  • subject + verb

Compare with a main clause question, where inversion happens:

• Kan de ses i kväll? = Can they meet tonight?

So:

• Hon hoppas att de kan ses i kväll.
• but: Kan de ses i kväll?

Yes, ses is formally the -s form of se. But in this context it is best understood idiomatically.

• se = to see
• ses can mean be seen, but very often with people it means see each other / meet

So here:

• de kan ses i kväll = they can see each other tonight / they can meet tonight

This is a very common Swedish way to express mutual meeting.

Yes, often you could.

• de kan ses i kväll = they can see each other / meet tonight
• de kan träffas i kväll = they can meet tonight

The difference is small in many everyday contexts.

A rough guide:

• ses can feel a little broader and more natural in casual speech
• träffas focuses more directly on the act of meeting

In this sentence, both work well, but ses is very idiomatic.

De is the subject form of they.

In the clause de kan ses i kväll:

• de = they
• kan = can

Since they is the subject of the verb kan, Swedish uses de, not dem.

Compare:

• De kan ses i kväll. = They can meet tonight.
• Jag ser dem. = I see them.

So:

• de = subject form
• dem = object form

In careful written-style speech, it may be pronounced close to de. But in everyday spoken Swedish, de and dem are very often both pronounced dom.

So although the sentence is written:

• de kan ses i kväll

many speakers would say something like:

• dom kan ses i kväll

This is normal in speech. In standard writing, though, you still usually write de here.

Yes, both i kväll and ikväll are used.

• i kväll is the more traditional separated form
• ikväll is also very common and accepted

They mean the same thing: tonight.

So these are both possible:

• de kan ses i kväll
• de kan ses ikväll

You will see both in modern Swedish.

A natural everyday pronunciation might sound approximately like:

• Hon längtar efter sin syster å hoppas att dom kan ses ikväll.

A few things change in normal speech:

• och is often pronounced like å
• de is often pronounced dom
• i kväll may sound like ikväll

So the written sentence stays formal and standard, but the spoken version is often a bit reduced.

It has two main verbs joined by och:

1. Hon längtar efter sin syster
2. (Hon) hoppas att de kan ses i kväll

The subject hon is stated only once, but it applies to both verbs:

• Hon längtar ... och hoppas ...

This is very common in Swedish, just like in English:

• She longs for her sister and hopes that they can meet tonight.

So the sentence structure is:

• subject + verb phrase + och
  • second verb phrase + subordinate clause