Когда зазвучала сирена, сотрудники сразу начали эвакуацию.

Зазвучала is the perfective past form of зазвучать, which means to begin sounding or to start being heard.

So:

• сирена звучала = the siren was sounding / used to sound
• сирена зазвучала = the siren started sounding

In this sentence, the important idea is the moment the siren began, because that event triggers what happens next.

Also, the prefix за- often gives this inceptive meaning: the start of an action.

Because the subject is сирена, which is:

• singular
• feminine

In the Russian past tense, verbs agree with the subject in gender and number:

• masculine: зазвучал
• feminine: зазвучала
• neuter: зазвучало
• plural: зазвучали

Since сирена is feminine singular, the verb is зазвучала.

Because Когда зазвучала сирена is a subordinate clause introduced by когда.

Russian normally separates subordinate clauses with commas, whether they come:

• before the main clause
• after the main clause
• in the middle of the sentence

So:

• Когда зазвучала сирена, сотрудники сразу начали эвакуацию.
• Сотрудники сразу начали эвакуацию, когда зазвучала сирена.

Both need the comma.

Here it means when, more specifically when/as soon as in the sense of a triggering event.

Because зазвучала is perfective and refers to the start of the siren, the clause points to a specific moment:

• Когда зазвучала сирена... = When the siren sounded / when the siren went off...

If the idea were more like while the siren was sounding, Russian would more naturally use an imperfective verb such as звучала.

Сотрудники is the nominative plural form of сотрудник.

It is nominative because it is the subject of the main clause:

• сотрудники начали = the employees began

It is plural because the sentence is talking about more than one employee.

Singular would be:

• сотрудник сразу начал эвакуацию = the employee immediately began the evacuation

Because эвакуацию is the accusative singular form, used for the direct object of начали.

They began what?

• эвакуацию

The noun эвакуация is feminine, and feminine nouns ending in -ия usually change like this in the accusative singular:

• nominative: эвакуация
• accusative: эвакуацию

So:

• начали эвакуацию = began the evacuation

Because начали эвакуацию specifically means began the evacuation. It focuses on the start of the process, not its completion.

That matches the structure of the sentence well:

• first, the siren starts
• then, the employees immediately start the evacuation process

If you used a verb meaning evacuated, the meaning would shift toward the action itself or its completion, not just its beginning.

Possibly, but the wording is a little more official and procedural.

начали эвакуацию most naturally suggests:

• they began the evacuation procedure
• they started carrying out an evacuation

If you wanted to say that the employees themselves started evacuating, Russian would more directly say something like:

• сотрудники сразу начали эвакуироваться

So the original sentence sounds more like they began the evacuation process than simply they started running out.

Because that is a very natural position for an adverb in Russian.

Сразу means immediately / right away, and putting it before the verb is common:

• сотрудники сразу начали эвакуацию

Other word orders are also possible, for example:

• сотрудники начали эвакуацию сразу
• сразу сотрудники начали эвакуацию

But the original order sounds neutral and natural.

Because the sentence narrates two past events:

1. зазвучала сирена — the siren started sounding
2. сотрудники начали эвакуацию — the employees began the evacuation

Russian commonly uses the past tense in both clauses when telling a sequence of events in the past.

Yes, both are perfective:

• зазвучала ← зазвучать
• начали ← начать

That makes sense because both verbs point to the beginning of actions:

• the siren started sounding
• the employees started the evacuation

Perfective verbs are often used for single, bounded events in a narrative. Here, both actions are presented as clear events that move the story forward.

Yes. Сотрудники usually means employees, staff members, or personnel. It often sounds a bit more formal or organizational than a general word like люди.

So this sentence sounds like something from:

• a workplace
• an office building
• a factory
• an official report or announcement

That fits well with эвакуация, which is also somewhat formal.

Yes. Russian word order is flexible, though different orders change the focus slightly.

For example:

• Когда зазвучала сирена, сотрудники сразу начали эвакуацию.
  Neutral: first the trigger, then the response.

• Сотрудники сразу начали эвакуацию, когда зазвучала сирена.
  Still correct, but the main action comes first in the sentence.

• Сразу, когда зазвучала сирена, сотрудники начали эвакуацию.
  Possible, with extra emphasis on immediately.

The original version is very natural and easy to follow.

Because Russian has no articles like a or the.

Whether something is definite or indefinite is usually understood from:

• context
• word order
• shared knowledge
• the situation being described

So сирена can mean:

• a siren
• the siren

And сотрудники can mean:

• employees
• the employees

In this sentence, context makes them sound definite in English, so the natural translation uses the.