Learn Hardcore Turkish: Yangın alarmı çalarsa acil çıkışa doğru koridoru hızlıca boşaltmak gerekir. - If the fire alarm rings, it’s necessary to quickly clear the corridor toward the emergency exit.

The suffix -(y)sa/-(y)se is the conditional marker meaning "if/should." In forms like this, Turkish typically uses the aorist first, then adds the conditional:

çal- (ring) + aorist -ar → çalar "rings/will ring (in general)"
plus conditional -sa → çalar-sa = çalarsa "if/should (it) ring"

So Yangın alarmı çalarsa means "If/should the fire alarm ring." For comparison:

çalıyorsa = "if it is ringing (right now)"
çalınca = "when/whenever it rings" (less hypothetical than -sa)
çaldığında = "when/at the time it rings" (formal/temporal)

Is a comma needed after the conditional clause?

It’s good style to put a comma after a sentence-initial conditional clause in Turkish:

Yangın alarmı çalarsa, ... You’ll also see it without a comma in short sentences; it isn’t strictly mandatory, but the comma improves readability.

Why does koridoru have the -u ending? I thought objects only take accusative if they’re definite.

Exactly: the accusative -(y)ı/-(y)i/-(y)u/-(y)ü marks a definite/specific direct object. Here, koridoru = "the corridor (we have in mind, e.g., in this building)." Without accusative, the object is indefinite/generic:

Koridoru boşaltmak gerekir. = "It is necessary to clear the corridor."
Koridor boşaltmak gerekir. = "It is necessary to clear corridor(s)." (generic/odd in this context) Because the corridor is specific and understood, accusative is natural here.

What exactly does acil çıkışa doğru mean? Why do we need both -a and doğru?

doğru is a postposition meaning "towards." It takes a complement in the dative case:

acil çıkış-a (to/towards the emergency exit) + doğru → acil çıkışa doğru "towards the emergency exit." Using only the dative (acil çıkışa) often implies motion “to” (reaching a destination), whereas ...a doğru emphasizes direction (“towards”), not necessarily arrival. Here it means "clear the corridor in the direction of the emergency exit."

Is the word order natural? Could I say the adverb and the directional phrase in other places?

Word order is flexible. The given order is very natural:

[condition] acil çıkışa doğru (direction) koridoru (object) hızlıca (manner) boşaltmak (verb) You can also say:
koridoru acil çıkışa doğru hızlıca boşaltmak gerekir
hızlıca koridoru acil çıkışa doğru boşaltmak gerekir All are acceptable; moving elements changes emphasis more than meaning.

Who is the subject of gerekir? Who is supposed to do the clearing?

This is an impersonal necessity construction: [V-mak] gerekir = "it is necessary to [V]." The doer is understood from context (people in the building). If you want to specify the agent, add a possessive to the -mak form:

koridoru … boşaltmamız gerekir = "we need to clear…"
… boşaltmanız gerekir = "you (pl.) need to…"
… boşaltmaları gerekir = "they need to…"

How does gerekir compare with gerekli, lazım, or zorunda?

… boşaltmak gerekir: neutral, general rule/necessity.
… boşaltmak gerekli/gereklidir: adjectival; also fine, sounds slightly more formal/statement-like.
… boşaltmak lazım: colloquial "it’s necessary/needed."
… boşaltmak zorunda(ız): strong obligation "must/have to."
Very formal passive: Koridorun … boşaltılması gerekmektedir/boşaltılmalıdır.

What’s the difference between boşaltmak and boşalmak?

boşaltmak (transitive) = "to empty/clear something." Takes an object: koridoru boşaltmak.
boşalmak (intransitive) = "to become empty/clear (by itself)." No object: Koridor boşaldı = "The corridor became empty." If you want "the corridor should be cleared" in passive: Koridor boşaltılmalıdır.

Is boşaltmak the right verb for evacuations? What about tahliye etmek?

boşaltmak is common and idiomatic for "clearing" a place of people or things. For formal safety language, tahliye etmek ("to evacuate") is also frequent:

Koridoru acil çıkışa doğru hızlıca tahliye etmek gerekir. Both are correct; tahliye sounds more technical/official.

Why use hızlıca and not hızlı or hızlı bir şekilde? Are there synonyms?

hızlı is an adjective; hızlıca is an adverb formed with -ca, meaning "quickly."
hızla is another adverb meaning "rapidly."
hızlı bir şekilde is a neutral paraphrase "in a fast way."
Other options: çabucak, ivedilikle (formal), derhal/hemen (immediately, focusing on time rather than speed). All would be understood; hızlıca/hızla are concise and natural here.

Could I say something like Yangın alarmı çalınca instead of çalarsa? Any nuance difference?

Yes.

çalarsa = "if/should [it] ring" (hypothetical, conditional).
çalınca = "when/whenever [it] rings" (eventive/temporal, less hypothetical).
Both are fine in instructions; çalarsa feels a bit more like a contingency ("in the event that").

Is Yangın alarmı a special noun compound? Why not yangının alarmı?

Yes, yangın alarmı is an indefinite noun compound ("fire alarm" as a type). In such compounds, the first noun is bare and the second carries 3rd person possessive (-ı/i/u/ü): alarm-ı.
Using yangının alarmı would be a definite possessed phrase ("the fire’s alarm"), which is not what we mean.

What tense/aspect is gerekir? Could I say gerekiyor or gerekmektedir?

gerekir is the aorist of gerekmek and states a general rule/necessity.

gerekiyor (progressive) = "is necessary (now/in progress)" and can feel more immediate.
gerekmektedir is a formal equivalent of gerekiyor.
All work; in written instructions, gerekir and gerekmektedir are common.

Does acil çıkışa doğru koridoru… mean we must reach the exit, or just move in that direction?

It emphasizes direction: "towards the emergency exit." It tells you to clear the corridor by moving people along it in that direction. It doesn’t explicitly say you must reach or pass through the exit, just the direction of evacuation.

Are there alternatives to alarm çalarsa for "if the alarm goes off"?

Yes:

Yangın alarmı öterse (beeps/sounds) — common with devices that "beep."
Yangın alarmı devreye girerse (goes into operation) — technical/formal.
Alarm verilirse (an alarm is given) — institutional phrasing. Still, alarm çalarsa is the most general and natural.

Why is it acil çıkışa doğru and not something like acil çıkış yönünde?

For movement, Turkish prefers the dative with a directional postposition:

… çıkışa doğru ("towards the exit") or … çıkış yönüne ("in the direction of the exit"). yönünde (locative) is more static ("in the direction of") and less natural for giving a movement instruction in this context. Both … çıkışa doğru and … çıkış yönüne are good here.