Learn Hardcore Turkish: Yastık kılıfını değiştirdim; çarşafla uyumlu oldu. - I changed the pillowcase; it matched the bedsheet.

yastık = pillow
kılıf = cover/case
yastık kılıfı = pillowcase. In a noun–noun compound, the second noun takes the compound/3rd-person possessive marker -(s)I → -ı. This does not mean “his/her” here; it just forms the compound.
Because it’s a specific direct object, we add the accusative -(y)I → -ı.
After a 3rd-person possessive ending, you insert the buffer -n- before a vowel-initial suffix.

So: kılıf + ı (compound poss) + n (buffer) + ı (accusative) → kılıfını.

Why use the accusative here? What changes if I say Yastık kılıfı değiştirdim?

Yastık kılıfını değiştirdim = “I changed the pillowcase” (a specific one both speaker and listener can identify).
Yastık kılıfı değiştirdim = “I changed a pillowcase” (non-specific/indefinite).
You can make the indefiniteness explicit with bir: Bir yastık kılıfı değiştirdim = “I changed a pillowcase.”

Does kılıfı mean “his/her case”? Are we implying possession?

No. In yastık kılıfı, the -ı on kılıfı is the standard compound marker used in noun–noun compounds; it doesn’t imply an owner. Examples:

mutfak bıçağı = kitchen knife
araba kapısı = car door The second noun carries -(s)I even when no person is owning it.

Could I say Yastık kılıfımı değiştirdim instead? What’s the difference from kılıfını?

kılıfımı = “my pillowcase” (1st-person possessive -ım
- accusative -ı).
kılıfını = “the pillowcase” (definite but not marked as “my”).
Both are fine; use kılıfımı if you want to emphasize it’s yours. In everyday talk, people often omit “my” when context makes it obvious.

What tense/person is değiştirdim, and how is it built?

Root: değiş- = change (intransitive)
Causative: değiştir- = change (something)
Past tense: -di → değiştirdi-
1st person singular: -m → değiştirdim = “I changed (it).”

What’s the difference between değişmek and değiştirmek?

değişmek (intransitive): “to change” (by itself)
- Ör. Fiyat değişti. = The price changed.
değiştirmek (transitive): “to change (something)”
- Ör. Yastık kılıfını değiştirdim. = I changed the pillowcase.

What does çarşafla mean, and is ile interchangeable?

çarşaf = bedsheet
-la/-le is the clitic form of ile = “with.”
So çarşafla = “with the sheet.”
You can write çarşaf ile (more formal) or the fused çarşafla (very common). After a vowel, it appears as -yla/-yle (e.g., arabayla).

Does uyumlu olmak always take ile/-la? Are there alternative patterns?

(X) (Y) ile uyumlu (olmak) is the standard pattern: “X is compatible/harmonious with Y.”
- Çarşafla uyumlu oldu.
An alternative is the verb uymak, which takes the dative -A:
- Çarşafa uydu. = “It matched the sheet.”

Why do we say uyumlu oldu instead of just uyumlu or uyumluydu?

uyumlu oldu = “became matching/compatible,” highlighting a change of state (after you changed the pillowcase).
uyumlu (present) would describe a current state without highlighting the “becoming.”
uyumluydu = “was matching (in the past),” a past state without the sense of change.

Could I replace uyumlu oldu with uydu? Any nuance difference?

Yes:

Çarşafla uyumlu oldu emphasizes the resulting harmonious look/compatibility.
Çarşafa uydu is a bit more direct and verb-like: “it matched/fit the sheet.”
Both are natural; choose based on whether you want an adjectival result (“compatible”) or the verb “fit/matched.”

Is the semicolon natural in Turkish here? What are other ways to connect the clauses?

A semicolon (;) is fine to link two closely related independent clauses. Alternatives:

Comma (informal style): Yastık kılıfını değiştirdim, çarşafla uyumlu oldu.
Conjunctions/adverbs:
- … ve çarşafla uyumlu oldu. (and)
- … böylece çarşafla uyumlu oldu. (thus)
- … artık çarşafla uyumlu. (now it’s compatible; no verb of becoming)

What is the subject of the second clause (çarşafla uyumlu oldu)? Why is it omitted?

The subject is understood as “it,” i.e., the pillowcase. Turkish often drops pronouns when person/number is clear from the verb or context. Here, after talking about the pillowcase, oldu (3rd singular) naturally refers back to it.

Can I change the word order? For example, can I say Yastık kılıfı çarşafla uyumlu oldu?

Yes. Some natural options:

Two sentences: Yastık kılıfını değiştirdim. Çarşafla uyumlu oldu.
Single sentence with explicit subject: Yastık kılıfı çarşafla uyumlu oldu. (states the result; doesn’t mention the changing action)
With a connector: Yastık kılıfını değiştirdim ve çarşafla uyumlu oldu. Turkish is flexible, but predicates typically come at the end of each clause.

Any pronunciation tips for tricky letters like ı, ç, and ş in this sentence?

ı (dotless i) in yastık, kılıfını: a close, central, unrounded vowel; think of a very short, relaxed “uh.”
i (dotted i): like English “ee.”
ç: “ch” as in “church” → çarşaf.
ş: “sh” as in “shoe” → çarşaf.
Word stress is typically on the final syllable: ya-STIK, kı-LI-fı-NI, çar-ŞAF-la, uyum-LU ol-DU.