Learn Hardcore Russian: За дверью стояла какая‑то женщина с маленькой собакой. - Behind the door there was some woman with a small dog.

за
- instrumental → location: “behind / beyond something” (where?)
  - за дверью = behind the door (static location)
за
- accusative → direction or purpose: “to go behind / beyond / to get / to pick up” (where to? for what?)
  - зайти за дверь = to go behind the door
  - зайти за молоком = to go (in order) to get milk

In this sentence, we’re describing where the woman was (a fixed position), so за takes the instrumental: за дверью.

Why does дверь become дверью? Where does that ending come from?

Дверь is a feminine noun of the “soft” (3rd) declension. Its instrumental singular ending is -ью.

Very short declension of дверь (sg.):

Nominative (who? what?): дверь
Genitive (of whom? of what?): двери
Dative (to whom? to what?): двери
Accusative (whom? what?): дверь
Instrumental (with whom? with what?): дверью
Prepositional (about whom? about what?): о двери

So дверью is simply the regular instrumental singular form of дверь. It’s used here because the preposition за requires the instrumental case for static location.

Why is the verb стояла used here instead of была (like “there was a woman”)?

Russian often prefers posture/position verbs like стоять (to stand), сидеть (to sit), лежать (to lie), висеть (to hang) instead of the general verb быть (to be) when describing where someone or something is.

За дверью стояла какая‑то женщина…
literally: Behind the door stood some woman…

If you say:

За дверью была какая‑то женщина…

it’s understandable, but it sounds less natural and more like you’re stating bare existence, not her posture. With стояла, you subtly paint a picture: she was standing there (perhaps waiting, maybe tense). It’s more vivid and idiomatic.

Why is it стояла and not стоял or стояло? How does the verb agree here?

In the Russian past tense, the verb agrees with the gender and number of the subject:

masculine sg.: стоял
feminine sg.: стояла
neuter sg.: стояло
plural: стояли

The subject here is женщина (a woman), which is feminine singular, so the verb must be стояла.

Examples:

За дверью стоял мужчина. – Behind the door stood a man.
За дверью стояло животное. – Behind the door stood an animal.
За дверью стояли люди. – Behind the door stood people.

What exactly does какая‑то mean? Is it like “some” or “some kind of”?

Какая‑то is an indefinite pronoun meaning roughly:

some, some kind of, a certain (but you don’t know or don’t want to specify who)

Nuances:

It shows that the speaker doesn’t know the woman or finds her unidentified / random:
- какая‑то женщина = some woman, a woman (I don’t know which one)

Compare:

женщина стояла за дверью – a woman was standing behind the door (neutral; just stating the fact)
какая‑то женщина стояла за дверью – some woman was standing behind the door (unknown / unexpected / random woman)

So какая‑то adds indefiniteness, sometimes with a light shade of “strange/unknown”.

Why is какая‑то written with a hyphen and not as two separate words?

Indefinite pronouns formed with ‑то are always hyphenated in modern Russian spelling:

какой‑то, какая‑то, какое‑то – some (kind of)
кто‑то – someone
что‑то – something
где‑то – somewhere

So какая‑то женщина must have the hyphen; какая то женщина would be a spelling mistake.

Why is it женщина (nominative) and not женщину or женщиной?

Женщина is the subject of the sentence, so it stands in the nominative case.

The structure is:

За дверью – adverbial phrase of place (“behind the door”)
стояла – verb (“was standing”)
какая‑то женщина с маленькой собакой – subject (“some woman with a small dog”)

In Russian, the subject often comes after the verb, especially when you start with a place or time expression, but it still stays in nominative:

На улице шёл дождь. – On the street was falling rain. → It was raining outside.
В комнате сидел мужчина. – In the room sat a man.

So even though женщина comes later in the sentence, it’s grammatically the subject and must be nominative.

What case is с маленькой собакой, and why is собакой in that form?

The phrase с маленькой собакой is in the instrumental case, governed by the preposition с.

с
- instrumental = with someone/something (together with)
  - с мамой – with mom
  - с другом – with a friend
  - с маленькой собакой – with a small dog

Собакой is the instrumental singular of собака.
The instrumental answers the question с кем? с чем? – with whom? with what?

So женщина с маленькой собакой literally is “a woman with a small dog.”

Why is the adjective маленькой in that specific form?

Маленькой is the instrumental feminine singular form of the adjective маленький (small).

It has to agree with the noun собакой in:

gender: feminine
number: singular
case: instrumental

So both change to instrumental feminine singular:

Nominative: маленькая собака – a small dog
Instrumental: с маленькой собакой – with a small dog

Russian adjectives always match their nouns in gender, number, and case. That’s why you see the same ‑ой ending on both маленькой and собакой here.

Why is it женщина с маленькой собакой (“a woman with a small dog”) and not женщина и маленькая собака (“a woman and a small dog”)?

С + instrumental (с маленькой собакой) shows accompaniment / possession in context: the dog is with the woman, probably hers, and they form one “group”.

If you said:

женщина и маленькая собака стояли за дверью

that would be grammatically correct, but the idea is slightly different: it sounds more like two separate subjects: “a woman and a small dog were standing behind the door”.

With женщина с маленькой собакой the focus is on one main figure (the woman), and the dog is just something she has with her.

Can we change the word order, for example: Какая‑то женщина с маленькой собакой стояла за дверью? Does the meaning change?

Yes, that word order is correct too:

За дверью стояла какая‑то женщина с маленькой собакой.
Какая‑то женщина с маленькой собакой стояла за дверью.

Both essentially mean the same thing. The difference is focus:

Starting with За дверью…: you first set the location (behind the door), then introduce who was there. It sounds a bit more narrative or descriptive.
Starting with Какая‑то женщина…: you focus first on who was there, then add where she was standing.

Russian word order is fairly flexible: as long as cases are correct, you can move parts around to change emphasis without breaking grammar.

What is the difference between за дверью and something like у двери?

These prepositional phrases describe different spatial relations:

за дверью – behind the door (on the other side of it, not visible from here)
у двери – by / at the door (near the door, next to it)

So:

За дверью стояла женщина – The woman is on the other side of the door.
У двери стояла женщина – The woman is near the door (on the same side as you, typically), perhaps in a hallway or room.

In your sentence, за дверью emphasizes that she’s out of sight, somewhere beyond the door.