Дежурная проверила, не остался ли кто‑нибудь в кабинете, и только потом вышла на лестницу.

Дежурная means the woman on duty or the female attendant/person in charge.

It comes from дежурный / дежурная, which can function either as:

• an adjective: дежурная медсестра = the nurse on duty
• or a noun by itself: дежурная = the woman on duty

In this sentence, the noun being described is omitted because it is understood from context.

Also, дежурная is feminine, so it tells you the person is female.

In the past tense, Russian verbs agree with the gender and number of the subject.

Since the subject is дежурная, which is feminine singular, the verbs are also feminine singular:

• проверил = he checked

• проверила = she checked

• вышел = he went out
• вышла = she went out

So the -а ending marks a feminine subject.

Both verbs describe completed actions in sequence:

• проверила = she checked / finished checking
• вышла = she went out / stepped out

The sentence is about a chain of finished events:

1. she checked
2. only after that, she went out

That is why perfective aspect is natural here.

If you used imperfective forms such as проверяла or выходила, the focus would shift more toward process, repetition, or background action, which would not fit this context as well.

This is a very common Russian pattern in indirect yes/no questions after verbs like:

• проверить = to check
• смотреть = to see
• спросить = to ask
• выяснить = to find out

So проверила, не остался ли кто‑нибудь... means:

• she checked whether anyone had remained...
• more literally: she checked if perhaps anyone was left...

Here, не ... ли often corresponds to English whether or if in this type of sentence. The не does not function like a straightforward English not. It is part of an idiomatic pattern.

Compare:

• Она спросила, не занят ли стул.
  = She asked whether the chair was taken.

• Он проверил, не забыл ли ключи.
  = He checked whether he had forgotten his keys.

So in your sentence, не остался ли is very natural Russian for whether anyone was still left in the room/office.

Ли is a particle used to form an indirect yes/no question.

In English, we usually use whether or if:

• She checked whether anyone was still in the office.

In Russian, ли usually comes after the word that is being questioned most directly.

Here:

• не остался ли кто‑нибудь...

This literally centers the question on did anyone remain?

You can think of ли as a marker that turns the clause into an embedded question.

Because the sentence is about uncertainty: whether anyone was left there.

Russian usually uses -нибудь forms in contexts like:

• questions
• conditions
• uncertainty
• possibilities

So:

• кто‑нибудь = anyone / somebody, if there happens to be one
• кто‑то = someone, usually a more definite or assumed person

Here, the speaker does not have a particular person in mind. The dежурная is checking whether there is anyone at all left in the office, so кто‑нибудь is the natural choice.

Because кто‑нибудь is grammatically treated as singular, and with кто-words Russian normally uses masculine singular agreement by default.

So:

• кто‑нибудь остался? = Did anyone stay behind?
• кто‑то пришёл. = Someone came.

Even if the person could actually be female, the grammar still usually defaults to masculine singular unless the real person is specifically identified later.

So остался agrees with кто‑нибудь, not with the possible real-life gender of the person.

Because в plus the prepositional case is used for location: in a place.

• кабинет = office / consulting room / study / room
• в кабинете = in the office / in the room

Since the meaning is static location, Russian uses the prepositional case:

• в кабинете
• в комнате
• в школе

If it were motion into the room, Russian would use the accusative:

• в кабинет = into the office

Только потом means only then, only after that, or and only afterward.

It emphasizes the sequence:

1. first she checked
2. only after doing that did she go out

So it adds a sense of not before that.

This makes the sentence sound more deliberate:

• she did not leave immediately
• she made sure nobody was still in the office first

In Russian, выйти на is a very common pattern meaning to go out onto some open area, platform, landing, balcony, street, etc.

Examples:

• выйти на улицу = go out onto the street
• выйти на балкон = go out onto the balcony
• выйти на сцену = go out onto the stage
• выйти на лестницу = go out onto the staircase / landing / stairwell

So на лестницу suggests she stepped out from the room onto the staircase area or landing.

Using в лестницу would be wrong here.

It can mean more than just the physical steps themselves.

Depending on context, лестница may refer to:

• the staircase
• the stairway
• the stairwell area
• sometimes the landing area around the stairs

So вышла на лестницу often means she stepped out onto the stairs / into the stairwell / onto the landing, not necessarily that she immediately started going up or down.

There are two reasons.

1. проверила, не остался ли кто‑нибудь в кабинете
   The comma separates the main clause from the subordinate indirect-question clause.

2. ..., и только потом вышла на лестницу
   The comma before и separates two coordinated actions:

   • she checked...
   • and only then she went out...

So the punctuation reflects the sentence structure:

• main clause
• subordinate clause
• second coordinated main clause

Yes, that is grammatically possible.

But there is a difference in feel:

• проверила, остался ли кто‑нибудь...
  = more neutral, simply checked whether anyone remained

• проверила, не остался ли кто‑нибудь...
  = more idiomatic in many contexts, often with the sense of made sure no one had been left behind

So the version with не is very natural because it reflects the practical concern: she wanted to make sure nobody was still there.

Yes: the word order is quite natural for narrative Russian.

• Дежурная проверила — the subject and completed action come first
• не остался ли кто‑нибудь в кабинете — the embedded question follows
• и только потом вышла на лестницу — the final action comes last, with только потом emphasizing sequence

Russian word order is flexible, but here the order is used to make the events feel orderly and clear:

• first the check
• then the exit

So this sentence is a good example of normal, idiomatic storytelling word order in Russian.