Learn Hardcore Russian: Одна девочка показывает свою любимую игрушку подруге. - One girl is showing her favorite toy to a female friend.

Одна девочка показывает… ≈ A (certain) girl is showing…
If you just say Девочка показывает свою любимую игрушку подруге, it usually means the girl is showing… (a specific girl known from context).

So одна helps introduce a new, previously unknown girl into the story. It’s not grammatically required, but it slightly changes the nuance from the girl to a girl / one girl.

Why is it одна and not один?

The numeral один / одна / одно / одни agrees in gender and number with the noun:

один мальчик (masculine) – one boy
одна девочка (feminine) – one girl
одно окно (neuter) – one window
одни очки (plural) – one pair of glasses

Because девочка is feminine singular, the correct form is одна.

Why is it свою and not её?

Russian has a special reflexive possessive pronoun свой that refers back to the subject of the sentence.

Свой means one’s own and changes form to match the thing owned, not the owner.
In this sentence, the subject is одна девочка, so свою means her own (the girl’s own).

You could say:

Одна девочка показывает свою любимую игрушку подруге.
→ One girl shows her own favorite toy to a (female) friend.

If you say её любимую игрушку, it is grammatically correct, but ambiguous: её can refer to some other woman/girl mentioned earlier, not necessarily the subject. Свою clearly ties the toy to the subject девочка.

Why exactly is the form свою любимую игрушку used? How do these endings work?

All three words are in the accusative singular feminine, because they are the direct object of the verb показывает (what is she showing?).

Base forms (nominative):

свой (possessive pronoun, masculine base form)
любимая (favorite, feminine)
игрушка (toy, feminine)

In the sentence:

свою – feminine accusative singular of свой
любимую – feminine accusative singular of любимая
игрушку – feminine accusative singular of игрушка

The pattern: adjective/pronoun + adjective + noun all agree in gender, number, and case:

своя любимая игрушка (nom., subject)
свою любимую игрушку (acc., direct object – what is shown)

Why is подруге in this form? Which case is it and why?

Подруге is dative singular of подруга (female friend).

The verb показывать / показать (to show) usually takes:

direct object in the accusative: игрушку – what is shown
indirect object in the dative: подруге – to whom it is shown

So:

показывать что? – игрушку (accusative)
показывать кому? – подруге (dative)

That is why we have подруге (ending -е), not подруга.

Is подруге always dative? It looks the same as the prepositional. How do we know?

The form подруге can be dative or prepositional; they are identical in the singular.

Dative: помогать подруге (to help a female friend)
Prepositional: говорить о подруге (to talk about a female friend)

Here we know it is dative because:

The verb показывать needs a recipient in dative (кому?).
There is no preposition (like о, в, на, etc.) that would normally signal the prepositional case.

So in показывает … подруге, context and verb pattern tell us it is dative: shows … to (a female) friend.

Can the word order change? For example, can we move подруге or свою любимую игрушку?

Yes. Russian word order is relatively flexible, and case endings keep the roles clear. Some possible variants:

Одна девочка подруге показывает свою любимую игрушку.
Одна девочка свою любимую игрушку показывает подруге.
Свою любимую игрушку одна девочка показывает подруге.

All of these are grammatically correct but change the emphasis:

Putting свою любимую игрушку first highlights the toy.
Putting подруге earlier can stress to the friend.

The original Одна девочка показывает свою любимую игрушку подруге is a neutral, natural order: subject – verb – direct object – indirect object.

Why is it показывает and not покажет? What aspect is this?

Показывает is:

Present tense
3rd person singular
Imperfective aspect (показывать)

Imperfective aspect is used for:

Ongoing processes (is showing),
Repeated/habitual actions,
Descriptions of what is happening in a scene.

So the sentence is like:

One girl is showing / shows her favorite toy to her friend.

If you used покажет (future, perfective показать):

Одна девочка покажет свою любимую игрушку подруге.
→ One girl will show her favorite toy to her friend (once, as a complete event).

So показывает describes the action in progress or generally, while покажет focuses on the completed result in the future.

How would the sentence change if the favorite toy belonged to the friend, not to the girl?

Then we would usually avoid свою, because свой always refers to the subject (девочка). To say the toy belongs to the friend (подруга), we would use её in the sense of her (the friend’s):

Одна девочка показывает её любимую игрушку подруге.
→ One girl is showing her (the friend’s) favorite toy to the (female) friend.

In real usage, such a sentence can still be a bit ambiguous without more context, so speakers often clarify:

Одна девочка показывает любимую игрушку своей подруги ей самой.
(literally: shows the favorite toy of her friend to her)

But grammatically, switching to её любимую игрушку is the key idea to show the toy is the friend’s, not the subject’s.

How would the sentence change if the friend was male instead of female?

The noun for a male friend is друг (friend, masculine).

We need the dative singular of друг:

Nominative: друг
Dative: другу

So the sentence becomes:

Одна девочка показывает свою любимую игрушку другу.
→ One girl is showing her favorite toy to (her) male friend.

Everything else stays the same.

How do we know that свою refers back to девочка and not подруга?

In Russian, the reflexive possessive свой always refers back to the subject of the clause, unless there is some special construction that changes this.

Here, the subject is одна девочка:

subject: одна девочка
verb: показывает
object: свою любимую игрушку
indirect object: подруге

So by default, свою refers to девочка. It does not refer to подруга, because подруга is not the subject, just the recipient (in dative). This is exactly why Russian uses свой: to avoid the ambiguity that её would have.

There is no word for a or the in the Russian sentence. How do we know it means one girl / a girl?

Russian does not have articles like English a / an / the. The specificity is shown by:

Context.
Words like один / одна / одно (one), этот / тот (this / that), etc.
Word order and what is already known in the conversation.

In this sentence, одна already plays a role similar to a / one:

Одна девочка показывает… → A (certain) girl is showing…

If we removed одна, the bare Девочка показывает… in isolation is usually understood as the girl is showing… or a girl is showing…, depending on context. Russian leaves the a/the distinction to interpretation from context rather than explicit words.