Begitu jendela dibuka, rasa sesak itu berkurang dan saya bisa menarik napas lebih tenang.

Here begitu means as soon as, once, or when. In the pattern Begitu + clause, ..., it introduces something that happens immediately after that event.

So:

Begitu jendela dibuka, ...
= As soon as the window was opened, ...

A learner may know begitu as like that / that way / so, and that meaning does exist too, but in this sentence it has the time-related meaning.

Dibuka is the passive form of buka.

• membuka = to open, in an active sense
• dibuka = to be opened

So:

• Begitu saya membuka jendela... = As soon as I opened the window...
• Begitu jendela dibuka... = As soon as the window was opened...

In your sentence, the focus is on the event of the window being opened, not on who opened it. Indonesian uses this kind of passive very naturally when the doer is unknown, obvious, or unimportant.

In that clause, jendela is the thing being talked about grammatically, and the person doing the action is left out.

This is very common in Indonesian. If the agent is not important, you can simply say:

jendela dibuka
= the window was opened

You do not need to say who opened it unless it matters.

If you wanted to include the agent, you could say something like:

Begitu jendela dibuka oleh saya...
but that sounds more formal and less natural than simply using an active clause:

Begitu saya membuka jendela...

Literally:

• rasa = feeling, sensation
• sesak = tight, constricted, stuffy, breathless

So rasa sesak means something like a feeling of tightness or a sensation of shortness of breath.

The exact nuance depends on context. It could refer to:

• chest tightness
• difficulty breathing
• a stuffy, oppressive feeling

In this sentence, it clearly relates to breathing, so the feeling of tightness / breathlessness is a good understanding.

Itu here points to a specific feeling already known in the context.

So:

• rasa sesak = a feeling of tightness
• rasa sesak itu = that feeling of tightness / the tightness

In Indonesian, itu often works a bit like English the or that, depending on context. It helps show that this is a specific sensation, not just a general idea.

Without itu, the phrase would feel more general or less specifically identified.

This is an important difference.

• kurang usually means less, not enough, or lacking
• berkurang means to decrease, to lessen, or to become less

So:

• rasa sesak itu kurang would not sound right here
• rasa sesak itu berkurang = the tightness decreased

In other words, berkurang is a verb describing a change.

Yes. Menarik napas is a very common expression meaning to breathe in, to take a breath, or sometimes more generally to breathe.

Literally it is:

• menarik = to pull
• napas = breath

So literally, to pull breath.

In this sentence, bisa menarik napas lebih tenang means the speaker could breathe more calmly, more comfortably, or with less distress.

Also note that napas is the standard modern spelling. You may also see nafas, but napas is preferred in standard Indonesian.

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

• bernapas = to breathe
• menarik napas = to draw breath / take a breath

Menarik napas can feel a bit more physical and vivid, which fits well when someone has just been struggling with tightness or breathlessness.

So:

• saya bisa bernapas lebih tenang = I could breathe more calmly
• saya bisa menarik napas lebih tenang = I could draw breath more calmly

The second one emphasizes the act of getting breath in.

Literally, lebih tenang means more calmly.

Indonesian often uses adjectives in a way that works like an adverb in English. So tenang is an adjective meaning calm, but after a verb it can describe how the action happens.

So:

menarik napas lebih tenang
= to breathe more calmly

In context, English might translate it more naturally as:

• breathe more calmly
• breathe more easily
• breathe with more ease

So yes, it literally means more calm, but in natural English the idea is often more comfortably / more easily.

Because Indonesian does not always need dengan to express manner.

You can say:

• menarik napas lebih tenang
• menarik napas dengan lebih tenang

Both are understandable. The version without dengan is more direct and very natural.

Indonesian often lets an adjective or adjective phrase follow the verb and function adverbially.

Literally, dan just means and.

However, in this sentence the meaning naturally suggests a result:

• the window was opened
• the tightness decreased
• therefore the speaker could breathe more calmly

Indonesian often leaves that cause-and-effect relationship to context rather than marking it explicitly with a special word. So dan is grammatically just and, but semantically it feels like and then / and as a result.

Yes. A few natural alternatives are:

• Begitu saya membuka jendela, rasa sesak itu berkurang dan saya bisa menarik napas lebih tenang.
  This makes it clear that I opened the window.

• Setelah jendela dibuka, rasa sesak itu berkurang dan saya bisa menarik napas lebih tenang.
  Setelah means after, which is a little less immediate than begitu.

• Begitu jendelanya dibuka, rasa sesak itu berkurang...
  jendelanya can sound a bit more specific, like the window or that window.

The original sentence is already very natural. The alternatives mainly change focus or nuance.