Learn Hardcore Turkish: O iyimser kalmayı seviyor, oysa kardeşi biraz kötümser düşünüyor. - She likes to stay optimistic, whereas her sister thinks a bit pessimistically.

Oysa means "whereas/however," introducing a contrast between two clauses. It often signals a stronger, more pointed contrast than ama ("but"). Halbuki is very close to oysa, a bit more conversational. You can also use oysa ki for a slightly more emphatic/literary feel. It typically appears at the start of the contrasting clause: "..., oysa ...".

Is that second o actually the pronoun again?

No. In "..., oysa ...", that o is just part of the single word oysa. There’s only one pronoun o in the sentence.

Do we need to keep the pronoun o at all?

No. Turkish is pro‑drop. You can say "İyimser kalmayı seviyor, oysa kardeşi biraz kötümser düşünüyor." The -yor ending already shows 3rd person singular.

Does o mean "he" or "she"? How do we know?

O is gender‑neutral ("he/she/it"). Context supplies gender if needed. Likewise, kardeşi means "his/her sibling" without specifying gender.

Why is it kalmayı and not kalmak after seviyor?

Because Turkish nominalizes verbs with -mA and then marks them as a direct object with the accusative -I; a buffer -y- appears between vowels:

kal- + -ma → kalma ("staying/remaining")
- -y- + -(ı) → kalmayı Verbs like sevmek, istemek, beklemek typically take this -mA(+y)I form for verb complements.

Could we say iyimser olmayı seviyor instead of iyimser kalmayı seviyor?

Yes. olmak = to be/become; kalmak = to remain.

iyimser olmayı seviyor: likes being/becoming optimistic (in general).
iyimser kalmayı seviyor: likes staying optimistic (maintaining that state, often despite challenges).

Why use seviyor and not the aorist sever?

Both are fine, with nuance:

seviyor: present progressive; commonly used for current or ongoing preferences.
sever: aorist/habitual; sounds more generic/timeless ("as a rule").

Who does kardeşi refer to? Is it definitely the same person's sibling?

By default, it refers back to the previously mentioned third person: "that person’s sibling." But it’s ambiguous in isolation. To be explicit, say onun kardeşi, use a name (Ali’nin kardeşi), or emphasize with kendi kardeşi ("his/her own sibling").

Why does the subject look like kardeşi with -i at the end? Is that accusative?

Here -i is not accusative; it’s the 3rd‑person possessive suffix (-(s)i), meaning "his/her." The word is nominative (no case ending) and functions as the subject:

kardeş
- -(s)i → kardeşi ("his/her sibling")

Could we drop the possessive and just say kardeş?

Not if you mean "his/her sibling." kardeş alone means "a/any sibling" and doesn’t link it to the person mentioned earlier. Use kardeşi (or onun kardeşi) for that meaning.

Why say kötümser düşünüyor instead of simply kardeşi biraz kötümser?

Both work, but they differ:

kardeşi biraz kötümser: the sibling is somewhat pessimistic (a trait).
kötümser düşünüyor: thinks in a somewhat pessimistic way (manner of thinking now or habitually).

Is kötümser düşünüyor idiomatic, or should it be kötümserce düşünüyor?

kötümser düşünüyor is idiomatic; adjectives often function adverbially. kötümserce düşünüyor (with -ce/-ca) is also correct and a bit more explicitly adverbial/formal. With biraz, both are natural.

Where should biraz go?

Default position is before what it modifies: kardeşi biraz kötümser düşünüyor. Postposing (... düşünüyor biraz) can occur in speech for afterthought emphasis, but the initial position is safest.

Can I replace oysa with ise to contrast the sibling?

Yes: O iyimser kalmayı seviyor; kardeşi ise biraz kötümser düşünüyor. Here ise is a contrastive clitic on the noun phrase "kardeşi." It’s a bit lighter than oysa and very common.

Any punctuation tips around oysa?

Use a comma or, more neatly, a semicolon before it:

O iyimser kalmayı seviyor; oysa kardeşi biraz kötümser düşünüyor. Starting a new sentence with Oysa is also fine; many writers omit the comma after it.