Один сотрудник надел маску, потому что дым в коридоре был слишком густым.

Here один сотрудник most naturally means one employee or a certain employee.

Russian has no articles like a or the, so words like один can sometimes help introduce a person as one / a certain person. In this sentence, it does not strongly emphasize the number unless the context makes that important.

So Один сотрудник can feel like:

• One employee
• An employee
• A certain employee

It depends on context.

This is a very common learner question.

In standard Russian:

• надеть что? = to put on something
  
  • надеть маску
  • надеть пальто
• одеть кого? = to dress someone
  
  • одеть ребёнка
  • одеть куклу

So:

• Он надел маску = He put on a mask.
• Он одел ребёнка = He dressed the child.

A common memory trick is:

• надеть = put on yourself / an item
• одеть = dress someone else

Because it is the direct object of надел.

The verb надеть takes a direct object in the accusative case:

• надеть что? → маску

Since маска is a feminine noun ending in -а, its accusative singular changes like this:

• nominative: маска
• accusative: маску

This is very regular for feminine nouns of this type.

Because потому что introduces a subordinate clause.

The sentence has two parts:

1. Один сотрудник надел маску
2. потому что дым в коридоре был слишком густым

In Russian, when a clause introduced by потому что follows the main clause, it is normally separated by a comma.

So this is standard punctuation:

• ..., потому что ...

Yes. Потому что is one of the most common ways to say because in Russian.

So:

• Он ушёл, потому что устал. = He left because he was tired.

It is very common in both speech and writing.

Other words can also mean because, such as так как, but потому что is the most straightforward everyday choice.

Because this phrase expresses location, not motion.

Russian uses:

• в + accusative for motion into a place
• в + prepositional for being in a place

So:

• идти в коридор = to go into the corridor
• быть в коридоре = to be in the corridor

In your sentence, the smoke is located in the corridor, so Russian uses the prepositional case:

• в коридоре

Был is the past tense of быть for a masculine singular subject.

The subject here is дым (smoke), which is masculine singular, so the verb agrees with it:

• masculine singular: был
• feminine singular: была
• neuter singular: было
• plural: были

Examples:

• Дым был густым.
• Комната была тёмной.
• Окно было открытым.
• Люди были уставшими.

So был matches дым.

This is about the predicate after быть.

In Russian, when an adjective is used after быть in the past or future, the instrumental case is very common:

• дым был густым
• чай был горячим
• он был уставшим

So густым is the instrumental singular masculine form of густой.

Why instrumental? Because Russian often uses the instrumental case for a predicate noun or adjective after forms of быть, especially when describing a state or characteristic.

You may also encounter nominative predicate adjectives in some contexts, but дым был слишком густым is very natural and standard.

Слишком means too in the sense of excessively.

So:

• слишком густым = too thick / too dense

Examples:

• слишком холодно = too cold
• слишком дорого = too expensive
• слишком поздно = too late

In this sentence, it shows that the smoke was dense enough to cause a problem.

Yes, дым means smoke.

Russian often uses singular mass nouns where English also uses a mass noun:

• дым = smoke
• вода = water
• воздух = air

So дым в коридоре means the smoke in the corridor or simply smoke in the corridor.

It is singular because smoke is being treated as an uncountable substance, not as separate units.

Надел is perfective.

The infinitive is надеть (perfective), and the past tense masculine singular form is надел.

Russian aspect matters a lot:

• надевал = was putting on / used to put on
• надел = put on, completed the action

In this sentence, the person actually put the mask on as a completed action, so the perfective is the natural choice.

The sentence is about a concrete event:

• the employee put on the mask
• because the smoke was too thick

So надел fits perfectly.

Yes. Russian word order is flexible, although each order gives a slightly different emphasis.

The neutral order here is:

• Один сотрудник надел маску, потому что дым в коридоре был слишком густым.

You could also say:

• Потому что дым в коридоре был слишком густым, один сотрудник надел маску.

This puts more focus on the reason first.

You could also move other parts around in certain contexts, but the original sentence sounds very natural and neutral.

Because Russian does not have articles.

English distinguishes:

• an employee
• the employee

Russian usually does not mark that difference with a separate word. Instead, meaning comes from context, word order, and sometimes words like один, этот, etc.

So:

• сотрудник can mean an employee or the employee
• дым can mean smoke or the smoke
• маску can mean a mask or the mask

In your sentence, context tells us how to understand them.