Pencere açılınca oda hemen aydınlandı.

Açılınca comes from:

• açıl- = to open / to be opened
• -ınca / -ince = when / once / as soon as

So açılınca means when it opened, when it was opened, or once it opened, depending on context.

The form is açıl + ınca. It is -ınca here, not -ince, because of vowel harmony.

Because açmak and açılmak are different verbs:

• açmak = to open something
  
  • example idea: I open the window
• açılmak = to open / to be opened
  
  • example idea: the window opens / is opened

In this sentence, pencere is the thing that opens, so Turkish naturally uses açılmak.

So:

• Pencere açınca would sound wrong or incomplete to most learners, because it suggests the window opened something
• Pencere açılınca correctly means when the window opened / was opened

This suffix creates a time clause. It often means:

• when
• once
• as soon as
• sometimes after

So Pencere açılınca... means something like:

• When the window opened...
• Once the window was opened...
• As soon as the window opened...

In this sentence, because of hemen (immediately), the meaning feels especially close to as soon as.

It can allow either reading, depending on context.

Açılmak often works like an intransitive/passive-type verb in Turkish, especially with things like:

• kapı = door
• pencere = window

So pencere açılınca can mean:

• when the window opened
• when the window was opened

If the speaker wanted to clearly mention the person doing the action, they would usually say something more explicit.

Turkish has no articles like English a / an / the.

So:

• pencere can mean a window or the window
• oda can mean a room or the room

Context tells you which one is meant. In this sentence, English often translates them as the window and the room, but Turkish does not need separate words for that.

Because oda is the subject of aydınlandı.

• aydınlandı = became bright / lit up
• What became bright? oda

So oda stays in the basic form.

You would use odayı only if room were a definite direct object, for example in a sentence like:

• Odayı aydınlattı. = He/She lit up the room.

But here the room is not an object; it is the thing that changes state.

Aydınlandı comes from aydınlanmak, which means to become bright, to light up, or to be illuminated.

Breakdown:

• aydınlan- = become illuminated / brighten
• -dı / -di / -du / -dü = past tense
• here it becomes aydınlandı = became bright / lit up

So this is a simple past tense verb.

It is not the same as:

• aydınlatmak = to illuminate something
• aydınlattı = made it bright / illuminated it

In your sentence, the room itself is changing state, so aydınlandı is the natural choice.

Hemen means immediately, right away, or at once.

It tells you how quickly the second action happened after the first one:

• Pencere açılınca = when the window opened
• oda hemen aydınlandı = the room immediately became bright

So the sentence emphasizes that the room got bright very quickly.

Yes, Turkish word order is flexible, but the given order is very natural.

The sentence is:

• Pencere açılınca = time clause first
• oda = subject
• hemen = adverb
• aydınlandı = main verb

This is a common pattern in Turkish: background information first, main verb at the end.

You could also hear variations such as:

• Pencere açılınca oda aydınlandı hemen.
• Oda pencere açılınca hemen aydınlandı.

But the original version sounds smooth and neutral.

Yes. A very common alternative is:

• Pencere açıldığında oda hemen aydınlandı.

Here açıldığında also means when it was opened / when it opened.

Very roughly:

• açılınca is often a bit shorter and lighter
• açıldığında can sound slightly more formal or explicit

Both are correct, and both are common.