Jag försöker andas lugnt när jag blir stressad.

After försöka (to try), Swedish often uses the infinitive without att in everyday language: Jag försöker andas.

You can also say Jag försöker att andas, and it is grammatical, but omitting att is very common and natural.

So both are possible:

• Jag försöker andas lugnt
• Jag försöker att andas lugnt

Andas is a deponent verb. That means it looks like an -s verb, but it has an active meaning, not a passive one.

So:

• att andas = to breathe
• jag andas = I breathe

Even though -s often makes Swedish verbs passive, that is not what is happening here. Andas is simply the normal dictionary form of the verb.

Here lugnt is being used as an adverb, meaning something like calmly.

The base adjective is:

• lugn = calm

But when Swedish adjectives are used adverbially, they often take the -t form:

• tala långsamt = speak slowly
• andas lugnt = breathe calmly

So lugnt is not describing a noun here. It is describing how the person breathes.

Bli means become or get, so:

• jag blir stressad = I get stressed / I become stressed
• jag är stressad = I am stressed

The sentence uses blir stressad because it focuses on the moment or situation in which stress happens to you.

So the nuance is:

• när jag blir stressad = when I get stressed
• när jag är stressad = when I am stressed

Both are possible, but they are slightly different in meaning.

Yes. In a sentence like this, när can often be understood as when or whenever, depending on context.

• Jag försöker andas lugnt när jag blir stressad can mean
  I try to breathe calmly when I get stressed
  or more generally
  I try to breathe calmly whenever I get stressed.

Because this sentence sounds like a general habit, many learners will naturally understand it as whenever.

Usually, no, not if you want the same meaning.

• när = when
• om = if

So:

• när jag blir stressad = when I get stressed
• om jag blir stressad = if I get stressed

The original sentence suggests that this is something that does happen, at least from time to time. Using om would make it sound more hypothetical.

Because there are two clauses, and each clause needs its own subject.

The sentence is:

• Jag försöker andas lugnt = main clause
• när jag blir stressad = subordinate clause

In English, you also repeat the subject:

• I try to breathe calmly when I get stressed

So the second jag is completely normal and necessary.

After när, Swedish uses subordinate clause word order, which here is the normal subject + verb order:

• när jag blir stressad

So:

• när = when
• jag = I
• blir = get/become
• stressad = stressed

There is no inversion here.

A useful comparison:

• Jag försöker andas lugnt när jag blir stressad.
• När jag blir stressad, försöker jag andas lugnt.

In the second version, the subordinate clause comes first, so the main clause changes word order to försöker jag.

In Swedish, you usually do not put a comma before a subordinate clause when the main clause comes first.

So this is normal:

• Jag försöker andas lugnt när jag blir stressad.

English punctuation habits often make learners expect a comma, but Swedish generally uses fewer commas in this kind of sentence.

If the när clause comes first, a comma is more common:

• När jag blir stressad, försöker jag andas lugnt.

Försöker is the present tense of försöka.

• försöka = to try
• försöker = try / am trying

In this sentence, the present tense describes a general habit or something the speaker usually does:

• Jag försöker andas lugnt när jag blir stressad = I try to breathe calmly when I get stressed

It is not necessarily happening right now; it can describe a repeated behavior.

Yes, very close. Stressad means stressed.

A useful distinction is:

• stressad = stressed, under stress
• stressig = stressful, hectic, causing stress

For example:

• Jag är stressad. = I am stressed.
• Det är en stressig dag. = It is a stressful/hectic day.

So in your sentence, stressad describes the person’s state, which is exactly the right choice.