Learn Hardcore Turkish: Siz çıkarken ışıkları kapatın; ben eskizleri masada bırakacağım. - Turn off the lights as you leave; I’ll leave the sketches on the table.

Siz is the plural or polite form of “you.” It’s used for addressing more than one person or a single person respectfully.
kapatın is the imperative for siz: stem kapat-
- plural/polite imperative -ın/-in/-un/-ün (here -ın by vowel harmony).
Casual singular would be Sen … kapat. Very formal/written instruction signs might use kapatınız.

Can I drop the pronouns Siz and Ben?

Yes. Turkish often omits subject pronouns because the verb endings show person: Çıkarken ışıkları kapatın; eskizleri masada bırakacağım.
Keeping Siz and Ben adds contrast/emphasis (“you…; I…”), which is useful here to assign tasks clearly.

What exactly does -ken in çıkarken mean, and why is it çıkarken (not “çıkken”)?

-ken makes a time clause meaning “while/as.”
With verbs, it attaches to the aorist stem: çık-ar-ken (the aorist of çıkmak is çıkar), hence the extra -ar-.
Compare: gel-ir-ken (from gelmek), okur-ken (from okumak).

What’s the difference between -ken and -ınca/-ince (e.g., çıkarken vs çıkınca)?

-ken = “while/as (in the process of).” Siz çıkarken = “as you are leaving/on your way out.”
-ınca/-ince = “when/once (upon).” Siz çıkınca often means “when you leave,” which in instructions commonly reads as “as you leave.” It can feel more event-pointed.
For turning off lights, both are heard; -ken highlights simultaneity during the act of leaving.

Why is there a semicolon, and could it be a comma or a period?

The semicolon links two closely related independent clauses; Turkish uses it much like English.
A comma is also fine: Siz çıkarken ışıkları kapatın, ben eskizleri masada bırakacağım.
A period is possible too; the semicolon simply shows a tighter connection.

Why is ışıkları in the accusative? Why not ışıklar?

Definite direct objects take the accusative -(y)I: ışık + lar + ı = ışıkları = “the lights.”
Here, the lights are contextually specific (the room’s lights), so the accusative is expected.
Vowel harmony chooses -ı because the last vowel of ışık is ı.

Can I say Kapatın ışıkları instead of Işıkları kapatın?

Yes. Both are grammatical.
Default neutral order is Object–Verb (Işıkları kapatın). Putting the object after the verb (Kapatın ışıkları) can add focus to “the lights (as opposed to something else).”

Why use kapatmak for lights instead of söndürmek?

kapatmak = “to turn off” (devices, switches): lights, TV, computer.
söndürmek = “to extinguish/put out,” prototypically for flames (candles, fire), but also used for lights in some contexts (ışığı söndür). For electric lights, kapatmak is the most neutral choice today.

What does -da in masada mean, and why not something like “üstünde”?

-DA is the locative case meaning “in/at/on,” depending on the noun: masa + da = masada.
In practice, masada here is read as “on the table.”
If you want to be explicit about “on top,” say masanın üstünde/üzerinde. Both are natural.

Why is the order Ben eskizleri masada bırakacağım and not Ben masada eskizleri bırakacağım?

Turkish is SOV, and the element right before the verb is the main focus.
In Ben eskizleri masada bırakacağım, the immediate pre-verb item is masada, so you’re emphasizing the place (“on the table”).
Ben masada eskizleri bırakacağım instead emphasizes the object (“the sketches” rather than something else).

How is bırakacağım formed, and how do you pronounce the ğ?

Morphology: bırak- (leave) + -AcAK (future) + -(y)Im (1sg) → bırakacağım.
Vowel harmony picks -acak (last vowel of the stem is ı) and -ım.
The k of -acak softens to ğ before a vowel-initial ending: bırakacak-ım → bırakacağım.
ğ is not a hard “g”; it lengthens or smooths the preceding vowel: roughly [bı-ra-ka-ca:a-m].

Could I say bırakırım instead of bırakacağım?

bırakırım (aorist) often conveys habitual action or a promise/volitional “Sure, I’ll leave (them).”
bırakacağım states a planned/decided future action. For a concrete plan like this, -AcAK is the default.

Why is eskizleri in the accusative?

It’s a definite plural direct object: eskiz + ler + i = eskizleri = “the sketches.”
If you mean an indefinite amount, don’t use the accusative and typically quantify: birkaç eskiz bırakacağım (“I’ll leave a few sketches.”)

What exactly does eskiz mean? Is there a synonym?

eskiz = “sketch” (loanword; common in art/design).
A near-synonym is taslak (“draft/rough draft”). In art contexts, eskiz is more specifically a drawing/sketch, while taslak can be broader (text, plan, or drawing).