Learn Hardcore Turkish: Eve girer girmez değil, çıkarken ışıkları kapat. - Don’t turn off the lights as soon as you enter, but when you are leaving.

It means “as soon as (someone) enters.” It’s a fixed idiom formed with the aorist (simple present) positive followed by the aorist negative of the same verb:

Pattern: V-(A/E)r V-(A/E)mez = “as soon as V”
Examples: gelir gelmez (as soon as s/he comes), gider gitmez (as soon as s/he goes), açar açmaz (as soon as s/he opens), oturur oturmaz (as soon as s/he sits), çıkar çıkmaz (as soon as s/he exits). Vowel harmony: the negative is -mez after front vowels (e, i, ö, ü) and -maz after back vowels (a, ı, o, u).

Why is değil used here? Is it negating the verb?

No. Değil is negating the time phrase, not the action. The structure is a contrastive correction: X değil, Y = “not X, but Y.” Here:

Eve girer girmez değil = “not (at the time) as soon as you enter,”
çıkarken ışıkları kapat = “turn the lights off when leaving.” Fully spelled out, you could say: Eve girer girmez ışıkları kapatma; çıkarken kapat.

Could I say Eve girince değil instead of Eve girer girmez değil?

You could, but the nuance changes:

girer girmez = “immediately upon entering,” very instantaneous.
girince = “when/once you enter,” more general, not necessarily immediate. Since the contrast here is about not doing it right away, girer girmez is the sharper, more natural choice.

Why are both parts of girer girmez in third person if the command addresses “you”?

Because V-(A/E)r V-(A/E)mez is a fixed idiomatic template that uses third-person aorist forms regardless of the actual subject. The understood subject usually matches the main clause. You can still specify a different subject explicitly:

Ben eve girer girmez ararım. (As soon as I enter, I call.)
Ali eve girer girmez çıkar. (As soon as Ali enters, he leaves.)

What does çıkarken mean exactly, and how is it different from çıkınca?

çıkarken = “while/as (you are) leaving.” It uses the aorist + -ken and highlights simultaneity or the process.
çıkınca = “when/once you leave (after you’ve left).” For switching off lights, çıkarken is natural because you do it as you are in the act of leaving. Çıkınca can sound like “after you have already left,” which doesn’t fit unless you can turn them off remotely.

Why is it Eve and not Evde or Evden?

Because girmek (“to enter”) takes the dative: you enter “into” a place.

Eve girmek = enter the house (to the house).
Evde = in the house.
Evden = from/out of the house (with çıkmak: evden çıkmak).

Why is ışıkları in the accusative? Could I say ışıklar instead?

Işıkları is accusative plural and marks a definite, specific set: “the lights” (of this place). In Turkish, definite direct objects take the accusative.

Işıkları kapat. = Turn off the lights. (definite)
Işıklar kapat. = ungrammatical as an object; ışıklar without case is typically a subject. An indefinite object would be bare and singular (e.g., Işık kapat. “Turn off a/some light”), but that’s not what’s meant here.

How polite is kapat? What are the formal alternatives?

kapat = 2nd person singular imperative, casual/familiar.
kapatın = 2nd person plural/formal, normal polite.
kapatınız = very formal/polite or instruction-manual style. You can add lütfen to soften: Lütfen ışıkları kapatın.

Could I say Işıkları söndür instead of Işıkları kapat?

Yes. söndür literally means “extinguish” and is used for flames, but it’s also common for lights. Both are fine:

Işıkları kapat. = turn the lights off (via a switch/device).
Işıkları söndür. = extinguish/turn the lights off (often slightly more “light-focused”). For devices in general, kapat is the default verb (“turn off the TV/computer/AC,” etc.).

What’s the role of the comma after değil? Could I use ama?

The comma separates the two contrasted instructions in X değil, Y. You may add ama for “but,” though it isn’t necessary:

Eve girer girmez değil, ama çıkarken ışıkları kapat. Another clean option is to split into two sentences:
Eve girer girmez ışıkları kapatma. Çıkarken kapat.

Can I change the word order, like Işıkları çıkarken kapat?

Yes. Turkish word order is flexible. These all work, with slight changes in emphasis:

Çıkarken ışıkları kapat. (neutral)
Işıkları çıkarken kapat. (focus on the object first)
Çıkarken kapat ışıkları. (more spoken style) Keep the time expression close to the verb for clarity.

Is there another way to say this without using değil?

Yes, use a negative imperative in the first part:

Eve girer girmez ışıkları kapatma; çıkarken kapat. or
Işıkları çıkarken kapat; girer girmez kapatma.

How do I form other -ken time expressions like çıkarken?

Attach -ken to:

the aorist: giderken, gelirken, çıkarken (habitual/general “when/while doing”), or
the present continuous: gidiyorken, geliyorken, çıkıyorken (emphasizes “while in the middle of doing”). Both mean “while/as,” with -yorken highlighting ongoingness more strongly.

Is Eve girer girmez değil fully grammatical even though the verb is missing?

Yes. It’s an acceptable ellipsis in the contrastive X değil, Y frame. The verb (kapat) is understood from the second clause. Expanded, it’s Eve girer girmez (ışıkları) kapatma; çıkarken (ışıkları) kapat. Ellipsis like this is natural in Turkish when the context makes the meaning clear.