Learn Hardcore Turkish: O sahile doğruca indi; ben ise parka döndüm. - He went straight down to the shore; I, however, returned to the park.

It marks a contrastive topic: “as for me,” “whereas I,” “on the other hand.” It’s not a full-blown “but” like ama; it simply contrasts the subject with what came before. It usually follows the word it contrasts (here, ben).

What’s the difference between ben ise and ben de?

ben ise = “whereas I / as for me” (contrastive: my action is different).
ben de = “I too / I also” (additive: my action is similar to someone else’s).
So changing ise to de would flip the meaning.

Can I write bense instead of ben ise? What about oysa ben?

bense (written as one word) is common and fully correct; it’s the clitic form of ise attached to ben. Meaning is the same as ben ise.
oysa ben is closer to “however, I…” and works as a clause-level connector; it feels a bit more like English “however/whereas” at the start of the clause.

Why is it sahil-e (to the shore) and not sahil-i?

Because motion “to/toward” takes the dative case -e/-a: sahil-e = “to the shore.” The accusative -i/-ı/-u/-ü marks a definite direct object: e.g., Sahil-i gördüm = “I saw the shore.”
Note: the dative harmonizes as -e or -a. sahil → sahil-e (front vowel), park → park-a (back vowel).

What does doğruca mean? How is it different from doğru, doğrudan, or direkt?

doğruca = “straight, straight to, directly (without stopping).”
doğru as a postposition: X’e doğru = “toward X” (direction, not necessarily arrival). As an adjective it means “true/correct.”
doğrudan / direkt (olarak) = “directly,” often interchangeable with doğruca.
Here, sahile doğruca indi = “(he/she) went straight down to the shore.”
Tip: sahile doğru fits verbs like yürümek (“walk toward”), but with inmek (“descend”), doğruca or doğrudan is more natural since arrival is implied.
Pronunciation: in doğruca, ğ lengthens the preceding vowel; c sounds like English “j” in “judge.”

Why use inmek (indi) instead of gitmek (gitti)?

inmek means “to go down/descend” or “to get off (a vehicle).” It implies a downward movement, which fits going down to a shore. gitmek is the general “to go,” without the vertical nuance.

Can doğruca move around in the sentence?

Yes. Common options include:

O doğruca sahile indi.
O sahile doğruca indi.
Both are natural; the first slightly foregrounds the manner (“straight/directly”), the second the destination. Placing doğruca at the very end is possible but marked for emphasis.

How are indi and döndüm formed?

indi = in- (stem “descend”) + -di (simple past, 3sg; harmonized here as -di).
döndüm = dön- (stem “turn/return”) + -dü (simple past, front rounded harmony) + -m (1sg).
Turkish past tense suffix appears as -di/-dı/-du/-dü by vowel harmony; person is on the very end.

Do I have to say o and ben? Can I drop them?

Turkish is pro-drop, so you can omit subject pronouns:

Sahile doğruca indi; parka döndüm.
This is grammatical, but you lose the explicit contrast marker ise unless you keep a host (e.g., bense):
Sahile doğruca indi; bense parka döndüm.

Why is it park-a and not something like parğa? Does the final k change?

Some words soften final p/ç/t/k to b/c/d/ğ before a vowel-initial suffix (e.g., renk → renge). But many loanwords like park don’t soften: it’s park-a, park-ı, not parğa/parğı.

Is the semicolon necessary here?

No, it’s stylistic. You could write:

O sahile doğruca indi, ben ise parka döndüm.
O sahile doğruca indi. Ben ise parka döndüm.
Or use a connector: O sahile doğruca indi, ama ben parka döndüm.
The semicolon neatly signals a tight contrast between two independent clauses.

Does sahil mean “beach”? Could I say plaj instead?

sahil = shore/coast/seaside in general.
kıyı = shore/edge (slightly more geographic).
plaj or kumsal = (sandy) beach.
So “to the beach” is plaj-a/kumsal-a; “to the shore/coast” is sahil-e/kıyı-ya.

Does döndüm mean “turned” or “returned”? Should I add geri?

dönmek can mean both “to turn” and “to return,” and context decides. parka döndüm is naturally understood as “I went back to the park.” If you want to make “back” explicit, add geri: parka geri döndüm.