Barnet börjar gråta när det blir för stökigt och högljutt i rummet.

Barnet is the definite singular form: the child.

• ett barn = a child
• barnet = the child

Swedish often adds the definite ending directly to the noun. Since barn is a neuter noun (ett word), the definite ending is -et.

After börja, Swedish usually uses the bare infinitive:

• börja gråta = start crying / begin to cry
• börja läsa = start reading

You can sometimes hear börja att, but börjar gråta is the most natural and common phrasing.

Gråta is the infinitive form, meaning to cry.

In this sentence, it comes after börjar, so together they mean:

• börjar gråta = starts to cry

Some related forms are:

• gråta = to cry
• gråter = cries / is crying
• grät = cried
• gråtit = cried

När means when.

In this sentence, it introduces the situation in which the child starts crying:

• när det blir för stökigt... = when it gets too chaotic...

This suggests something that happens whenever that situation occurs, or at the time that it occurs.

Compare:

• när = when
• om = if

So om det blir för stökigt would mean if it gets too chaotic, which is slightly different.

Here, det is most naturally an impersonal/dummy subject, much like English it in it gets too noisy.

It does not mainly mean the child. The clause is talking about the general situation or atmosphere:

• det blir för stökigt och högljutt = it gets too chaotic and loud

So this det is more about the environment/situation than a specific thing.

Blir means becomes / gets, while är means is.

So:

• det blir för stökigt = it gets too chaotic
• det är för stökigt = it is too chaotic

Using blir focuses on a change or development: the room reaches a point where it becomes too chaotic and loud.

Here, för means too.

So:

• för stökigt = too chaotic / too messy / too rowdy
• för högljutt = too loud / too noisy

This is a very common Swedish pattern:

• för dyrt = too expensive
• för kallt = too cold
• för svårt = too difficult

They end in -t because they are agreeing with det, which takes neuter singular agreement.

These are predicative adjectives after blir:

• det blir stökigt
• det blir högljutt

That -t is the normal neuter ending for many adjectives.

A couple of comparisons:

• en situation är stökig
• ett rum är stökigt

Also note that högljudd changes form a bit:

• högljudd → högljutt

They are related, but not identical:

• stökigt = chaotic, disorderly, rowdy, messy in an active or social sense
• högljutt = loud, noisy

So the sentence is not just saying the room is loud. It is saying that it is both:

• too chaotic/disruptive
• and too noisy

Together they create a stronger picture of an overstimulating environment.

Because när det blir för stökigt... is a subordinate clause, not a question.

In Swedish:

• Questions often invert the verb and subject: Blir det för stökigt?
• Subordinate clauses keep normal subject-verb order: när det blir för stökigt

So:

• när det blir... = when it gets...
• blir det...? = does it get...?

I rummet means in the room or inside the room.

For an enclosed space like a room, i is the natural preposition:

• i rummet = in the room
• i huset = in the house
• i bilen = in the car

So this part simply tells you where the noise and chaos are happening.