Yastık kılıfı temiz olunca daha rahat uyuyorum.

Yastık kılıfı means pillowcase.

This is a very common Turkish noun pattern:
noun + noun-(s)I

So:

• yastık = pillow
• kılıf = cover, case
• yastık kılıfı = pillow cover / pillowcase

The -ı on kılıfı is not a direct object ending here. It is the 3rd person possessive suffix, which is used in this kind of noun compound. In English we do not show this structure the same way, so it can feel strange at first.

Compare:

• kitap kapağı = book cover
• araba kapısı = car door
• telefon ekranı = phone screen

So yastık kılıfı is just the normal way to say pillowcase.

Not in this sentence.

Although -ı / -i / -u / -ü can be a possessive ending meaning his/her/its, in noun compounds like yastık kılıfı, it is just part of the compound structure.

So:

• yastık kılıfı = pillowcase
  not necessarily
• its pillow cover

Context matters. Here it is simply the dictionary-style compound meaning pillowcase.

Olunca means something like:

• when it is
• when it becomes
• once it is
• sometimes even if it is, depending on context

It comes from:

• olmak = to be / to become
• -ınca / -ince / -unca / -ünce = when, once, after

So:

• temiz olunca = when it is clean / when it becomes clean

In this sentence, the idea is: When the pillowcase is clean, I sleep more comfortably.

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

• temiz olunca = when it is clean / when it becomes clean
• temizken = when it is clean / while it is clean

Olunca often gives a sense of a condition being met or a state resulting in something:

• When the pillowcase is clean, I sleep better.

-ken is more like while/when in that state:

• Yastık kılıfı temizken daha rahat uyuyorum.
  = I sleep more comfortably when the pillowcase is clean.

Both are natural. Temiz olunca is very common and conversational.

Daha rahat means more comfortably or more easily here.

Word by word:

• daha = more
• rahat = comfortable, comfortably, at ease

In Turkish, adjectives often also function like adverbs depending on context. So rahat can mean:

• comfortable as an adjective
• comfortably / with ease as an adverb-like word

Since it describes uyuyorum (I sleep), the sense here is adverbial:

• daha rahat uyuyorum = I sleep more comfortably

This is a very common question.

• uyuyorum is the present continuous
• uyurum is the aorist, often used for habits or general truths

In English, you might expect a habitual form, so uyurum may seem more logical. But in Turkish, the present continuous is often used for personal habits and repeated experiences in everyday speech.

So:

• Yastık kılıfı temiz olunca daha rahat uyuyorum.
  = When the pillowcase is clean, I sleep more comfortably.

This sounds very natural.

If you said:

• Yastık kılıfı temiz olunca daha rahat uyurum.

that would also be understandable, but it sounds a bit more general or characteristic, and often a little less conversational in this context.

The subject is I, but Turkish often leaves subject pronouns out because the verb ending already shows the person.

• uyu-yor-um
• -um = I

So uyuyorum already means I am sleeping / I sleep.

Turkish does this all the time:

• Geliyorum = I am coming
• Biliyorum = I know
• Uyuyorum = I am sleeping / I sleep

You could add ben for emphasis:

• Ben yastık kılıfı temiz olunca daha rahat uyuyorum.

But normally it is not necessary.

Yes, the subject is understood from context.

In Yastık kılıfı temiz olunca, the subject of olunca is yastık kılıfı.

So the full sense is:

• When the pillowcase is clean...

Turkish often avoids repeating things that are already clear. Nothing important is missing; it is just normal Turkish structure.

Yes. That is one reason olunca is flexible.

Because olmak can mean both to be and to become depending on context, temiz olunca can suggest either:

• when it is clean
• when it becomes clean

In this sentence, English would usually translate it as when the pillowcase is clean, because that sounds most natural. But the Turkish expression can also carry the idea of once it gets clean.

Yes, absolutely.

• temiz olunca
• temiz olduğunda

Both can mean when it is clean.

The difference is mostly style and feel:

• olunca = often more conversational, compact, natural in everyday speech
• olduğunda = a bit more explicit, sometimes slightly more formal or written

So both are correct:

• Yastık kılıfı temiz olunca daha rahat uyuyorum.
• Yastık kılıfı temiz olduğunda daha rahat uyuyorum.

The first one feels especially natural in casual speech.

Turkish often uses -ınca/-ince for situations where English might say either when or if, depending on meaning.

In this sentence, the idea is not a strong hypothetical like:

• If the pillowcase were clean, I would sleep better

Instead, it is more like a regular situation:

• When the pillowcase is clean, I sleep more comfortably

So olunca fits very well.

If you wanted a more explicit if, you could use -sa/-se:

• Yastık kılıfı temizse daha rahat uyuyorum.

This also works and means:

• If the pillowcase is clean, I sleep more comfortably.

Compared with temiz olunca, temizse sounds a bit more directly conditional.

No, Turkish word order is flexible, but some orders sound more natural than others.

The original sentence:

• Yastık kılıfı temiz olunca daha rahat uyuyorum.

is very natural because it gives the condition first, then the result.

You could also hear:

• Daha rahat uyuyorum yastık kılıfı temiz olunca.

This is possible, but it feels more marked or conversational, depending on intonation.

A very common neutral order in Turkish is:

• background or condition first
• main action later

So the original sentence is a good model.

Turkish does not have articles like the and a/an the way English does.

So:

• yastık kılıfı can mean the pillowcase, a pillowcase, or pillowcases in general, depending on context

Here, the meaning is understood from the sentence as a general statement about the pillowcase / my pillowcase in context.

This is very normal in Turkish, and learners gradually get used to letting context do that job.

Uyuyorum comes from the verb uyumak = to sleep.

Formation:

• verb stem: uyu-
• present continuous marker: -yor
• 1st person singular: -um

So:

• uyu + yor + um → uyuyorum

The repeated vowel sound is normal here. Turkish often inserts or keeps sounds in ways that make pronunciation smoother.

Other examples:

• okuyorum = I am reading
• yürüyorum = I am walking
• uyuyorum = I am sleeping

So even if it looks a little unusual at first, it is a regular and common form.

Yes, definitely.

This sentence does not only describe one single occasion. It naturally sounds like a repeated personal experience or preference:

• When the pillowcase is clean, I sleep more comfortably.

That is why uyuyorum works well here. In everyday Turkish, it can describe something that is regularly true for the speaker, not only what is happening right now.