Learn Hardcore Turkish: Bu sırada yangın alarmı denendi, herkes sakin kaldı. - Meanwhile, the fire alarm was tested; everyone stayed calm.

It means at this time, during this period, or meanwhile. It anchors the clause to a time frame already understood from context. Using O sırada shifts the reference a bit farther away (more like at that time (then)), while Bu sırada feels more immediate to the current narrative.

Can I replace Bu sırada with Bu arada or Bu esnada?

Bu esnada is a near-synonym of Bu sırada and works fine.
Bu arada can mean in the meantime, but very often means by the way as a discourse aside. If you want a pure time reference, Bu sırada/Bu esnada is safer.

Why does yangın alarmı end with -ı? Is that the accusative?

No. In yangın alarmı (fire alarm), the -ı is the 3rd person possessive marker that forms an indefinite noun compound: yangın (fire) + alarm (alarm) + -ı → yangın alarmı. It’s not the accusative. If it were accusative, you would see an extra -n-: yangın alarmını.

So what is the subject of denendi here?

The subject is yangın alarmı (the fire alarm). Because the verb is passive, the thing acted upon (the alarm) becomes the grammatical subject: yangın alarmı denendi = the fire alarm was tested.

Is denendi passive? How is it built?

Yes. It comes from denemek (to try/test). Passive is formed with -n-: denen- (to be tested) + past tense -di → denendi (was tested).

How do you say was tested in Turkish—do you need a separate “to be” verb?

No separate auxiliary is used. The passive verb plus past -di already encodes was …-ed: denen- (be tested) + -di → denendi.

Could I make it active, like Yangın alarmını denediler?

Yes. That’s the active form: (Onlar) (they) yangın alarmını denediler (they tested the fire alarm). Note the accusative -ı: alarmını. The original sentence is passive and leaves the agent unspecified.

What tense are denendi and kaldı?

Both are simple past (-di past). Denendi = was tested; kaldı = remained/stayed.

Why sakin kaldı and not sakin oldu?

sakin kalmak = to remain/keep calm (focus on continuity).
sakin olmak = to become calm (focus on change of state). Here, the idea is they did not panic; they maintained calmness, so sakin kaldı fits.

Why is the verb singular with herkes?

Herkes (everyone) is grammatically singular in Turkish, so the verb is 3rd person singular: herkes sakin kaldı. In casual speech you may hear plural agreement, but standard usage is singular.

Can I change the word order?

Yes, within limits:

Bu sırada yangın alarmı denendi (neutral, very natural).
Yangın alarmı bu sırada denendi (also fine; slight emphasis on the time).
Bu sırada denendi yangın alarmı (possible for focus, but less neutral). Turkish allows flexible order for emphasis, but keeping adverb–subject–verb here is the most natural.

Is the comma between the two clauses enough, or should I use a conjunction?

The comma is fine for asyndetic coordination. You could add:

… denendi ve herkes sakin kaldı (and).
… denendi ama herkes sakin kaldı (but).
… denendi; herkes sakin kaldı (semicolon is also acceptable).

Does yangın alarmı mean a specific alarm (the) or any alarm (a)?

Turkish doesn’t mark definiteness the way English does. Yangın alarmı can map to either the fire alarm or a fire alarm, depending on context. If you need to be explicit:

Specific: bu/şu/o yangın alarmı (this/that).
Non-specific: bir yangın alarmı (a).

What would yangın alarmı çaldı mean compared to denendi?

yangın alarmı çaldı = the fire alarm rang/sounded (it went off).
yangın alarmı denendi = the fire alarm was tested (someone tested it).

Are there synonyms for denendi that fit this context?

Common alternatives:

test edildi (was tested).
alarm denemesi yapıldı (a test/try of the alarm was carried out).
In drill contexts: tatbikat yapıldı (a drill was conducted), though that refers to the whole drill, not just the device.

Can I use sırasında instead of sırada?

Use -sırasında with a specific noun: tatbikat sırasında (during the drill), film sırasında (during the movie). Bu sırada is a fixed time adverbial; bu sırasında is not used.

Any quick pronunciation or spelling tips here?

sakin (calm) vs sakın (don’t!/beware): different vowels and meanings.
alarmı uses back vowel -ı (3rd person possessive) due to vowel harmony; it’s not alarımı.
yangın has a back ı in the last syllable: yan-gın.