Hon ser lycklig ut när hon sitter med familjen på balkongen, och hennes mamma blir också lycklig av det.

Because se ... ut means look / appear rather than be.

• Hon är lycklig = she is happy
• Hon ser lycklig ut = she looks happy / seems happy

So this sentence is describing how she appears, not directly stating her inner emotional state as a fact.

Because se ut is a particle verb. In Swedish main clauses, the finite verb usually comes early in the sentence, but the particle often stays later.

So:

• Hon ser lycklig ut
• not Hon ut ser lycklig

This is very common in Swedish:

• Hon ser trött ut = She looks tired
• Det går bra till slut = It works out well in the end

Because när hon sitter med familjen på balkongen is a subordinate clause.

In Swedish subordinate clauses, the word order is usually:

subordinator + subject + verb

So:

• när hon sitter = when she is sitting

The inverted order sitter hon is used in main clauses and questions, not here.

Compare:

• När sitter hon på balkongen? = When is she sitting on the balcony?
• ...när hon sitter på balkongen = ...when she is sitting on the balcony

Swedish often uses posture verbs more naturally than English does.

Here, sitter means is sitting / is seated. Swedish likes to describe a person’s physical position:

• sitta = sit
• stå = stand
• ligga = lie

So när hon sitter med familjen på balkongen sounds natural and specific: it paints the scene. English might sometimes just say when she is with her family, but Swedish often prefers the posture verb if the position matters or is part of the image.

Because the context already makes it clear which family is meant.

Swedish often uses the definite form when something is understood from context:

• familjen = the family

In this sentence, it naturally means her family, even though that is not spelled out with a possessive.

Other versions are possible, but they sound slightly different:

• med hennes familj = with her family
  
  • more explicit
• med sin familj = with her own family
  
  • also possible in many contexts, especially if you want to show the connection clearly

Here, med familjen is just a smooth, natural choice.

Because Swedish uses på for being on a balcony.

A balcony is treated as an open platform or surface area, so Swedish says:

• på balkongen = on the balcony

Using i would sound wrong for normal location here.

This is similar to other Swedish location patterns where English learners sometimes expect in, but Swedish uses on.

Because sin/sitt/sina can only refer back to the subject of the same clause.

In the second clause:

hennes mamma blir också lycklig av det

the subject is mamma. If you used sin mamma, it would wrongly suggest that the mother is somehow being linked to herself as the possessor.

So Swedish uses hennes here to mean her mother = the mother of the girl mentioned earlier.

This is a very important Swedish rule:

• Hon älskar sin mamma = She loves her own mother
  
  • sin refers back to hon, the subject of that same clause
• Hennes mamma blir lycklig = Her mother becomes happy
  
  • here hennes refers to someone else already mentioned

Because bli means become / get, so it shows a change of state.

• är lycklig = is happy
• blir lycklig = becomes happy / gets happy

In this sentence, the mother reacts to the situation, so blir lycklig is very natural: she becomes happy because of it.

Here av det means because of that or from that.

So:

• hennes mamma blir också lycklig av det
  = her mother also becomes happy because of that

The det does not refer to just one noun. It refers to the whole situation described earlier: seeing her happy with the family on the balcony.

Because in a normal Swedish main clause, the finite verb usually comes in second position. After that, adverbs like också often come next.

So:

• hennes mamma blir också lycklig

This is the neutral, natural word order.

If you move också, the emphasis can change. But this placement is the most straightforward one for this sentence.

Not always. In Swedish, a comma before och is often optional.

Here it is used to separate two fairly long clauses:

• Hon ser lycklig ut när hon sitter med familjen på balkongen
• och hennes mamma blir också lycklig av det

So the comma helps readability, but many writers would also write the sentence without it.

Because lycklig is a predicative adjective here, and both times it describes a singular common-gender noun/person:

• hon → lycklig
• mamma → lycklig

So no change is needed.

You would see different forms in other contexts, for example:

• ett lyckligt barn = a happy child
• de är lyckliga = they are happy

But in this sentence, lycklig is the correct form both times.