Learn Hardcore Turkish: Ekran kilidi aktif, şifreyi girince hemen açılıyor. - The screen lock is active; when I enter the password, it opens immediately.

Because this is an indefinite compound noun (belirtisiz isim tamlaması): Noun1 + Noun2 with a 3rd person possessive on the second noun. So ekran kilidi literally means “screen’s lock” as a type of lock, just like:

araba kapısı (car door)
banka kartı (bank card) Using bare kilit here would be ungrammatical; the head noun takes the -I possessive: kilidi.

Could I say ekranın kilidi instead?

You can, but it’s a different structure (definite compound: genitive + possessed). Ekranın kilidi points to the lock of a specific screen (“the screen’s lock”), whereas ekran kilidi is the general feature name “screen lock.” In UI language and general talk about the feature, ekran kilidi is standard.

Where is the verb “is” in Ekran kilidi aktif?

Turkish drops the present “to be” in 3rd person. Ekran kilidi aktif literally “screen lock active” = “The screen lock is active.” You can add -dir for formality or certainty: Ekran kilidi aktiftir, but it’s not required in everyday speech.

What does the -yi in şifreyi do?

It’s the accusative marker -ı/-i/-u/-ü showing a specific/definite direct object. Şifreyi = “the password.” The buffer -y- appears because şifre ends in a vowel. Without accusative, şifre girince would mean “when a password (in general) is entered,” which is less natural here.

Would şifreni girince work, and what does it mean?

Yes: şifreni means “your password” (2nd person singular possessive + accusative). So şifreni girince = “when you enter your password.” For polite/plural “your,” use şifrenizi.

What exactly does girince mean, and how is it formed?

It’s the temporal adverbial suffix -(y)ince/ınca/ünce/unca meaning “when/once/whenever (someone) does X.” Form: verb stem + the suffix with vowel harmony:

gir-
- -ince → girince (when (someone) enters)
gel-
- -ince → gelince
ara-
- -yınca → arayınca (buffer -y- after a vowel)

Does girince have to share its subject with the main clause?

Typically, an unmarked -ince clause shares its subject with the main clause. In everyday speech, though, people often use a generic “you/one” in the -ince clause that triggers an action on something else, as in your sentence. If you want to be explicit:

Sen şifreyi girince, ekran kilidi açılıyor. Or use the subject-marking time form -DIğIndA:
Şifreyi girdiğinde (when you enter)
Şifreyi girdiğimde (when I enter)
Şifreyi girdiğimizde (when we enter)

Why açılıyor and not açılır or açıyor?

açılıyor is present continuous of açılmak and in colloquial Turkish it often describes repeated or typical outcomes: “it opens (when that happens).”
açılır (aorist) also works and sounds more general/instructional (“it opens” as a rule).
açıyor would be transitive (“opens [something]”) and doesn’t fit, because the lock isn’t opening another thing; it itself is opening.

What’s the difference between açmak and açılmak here?

açmak = to open something (transitive).
açılmak = to open / to be opened (intransitive/passive-like). So you say: Kilit açılıyor (The lock opens / is opening), not Kilit açıyor (which would mean “The lock opens [something]”).

Who or what is the subject of açılıyor? Where is “it”?

It’s understood from context. Turkish drops subject pronouns when clear. Here, ekran kilidi from the first clause is the understood subject of açılıyor. You could repeat it: Ekran kilidi, şifreyi girince hemen açılıyor, but it’s not necessary.

What does hemen add, and where should it go?

Hemen means “immediately/at once.” It most naturally appears right before the verb it modifies: hemen açılıyor. Synonyms include anında (also common) and derhal (more formal/urgent): şifreyi girince anında açılıyor.

Why use aktif instead of açık or etkin?

All can be seen:

aktif is common in tech/UI for “enabled/active.”
etkin is a more “native” alternative to the loanword aktif.
açık often means “on/open,” but with kilit it can be ambiguous (it can sound like “unlocked”). For clarity about a feature being enabled, aktif/etkin is safer: Ekran kilidi aktif/etkin.

Can I reorder the clauses?

Yes. Fronting the “when” clause is very natural:

Şifreyi girince, ekran kilidi hemen açılıyor. Keep a comma after the fronted -ince clause for readability. You could also separate the two main statements with a semicolon or period: Ekran kilidi aktif; şifreyi girince hemen açılıyor.

Is girince closer to “when,” “once,” or “as soon as”?

It can cover all of those depending on context. By default it means “when/once,” and with hemen it conveys “as soon as”: şifreyi girince hemen açılıyor ≈ “as soon as you enter the password, it opens.”

Could I say şifre girince without the accusative?

You can, but it suggests an indefinite reading (“when a password is entered”), which is a bit impersonal. If you want that impersonal passive style, a clearer option is Şifre girildiğinde, … (“when the password is entered”). For the usual “when you enter the password,” şifreyi girince is best.