Сестра оформила возврат, потому что обмен на другой размер оказался невозможен.

Because the subject is сестра, which is a feminine noun.

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

• оформил = he processed / arranged
• оформила = she processed / arranged
• оформило = it processed / arranged
• оформили = they processed / arranged

So Сестра оформила возврат means The sister processed/arranged the return.

Оформить often means to process, arrange, make official, fill out properly.
So оформить возврат is not just to return something, but more specifically to process a return or to arrange the return formally.

This is common in store/customer-service language.

Compare:

• вернуть товар = to return the item
• оформить возврат = to process the return / complete the return procedure

So this sentence sounds like a retail or service situation.

Russian often uses a verb + noun combination where English might prefer a simple verb.

Here:

• оформила возврат = processed a return

Instead of:

• вернула товар = returned the item

The noun возврат focuses on the return as a procedure or transaction, not just the physical act of giving something back.

Because потому что means because and introduces a subordinate clause.

So the sentence has two parts:

• Сестра оформила возврат
• потому что обмен на другой размер оказался невозможен

Russian normally puts a comma before потому что, just like English often separates the main clause from the reason clause.

Обмен is in the nominative case because it is the subject of the second clause:

• обмен ... оказался невозможен
• the exchange ... turned out to be impossible

Here, обмен is the thing that turned out to be impossible, so it is the grammatical subject.

The preposition на is commonly used with обмен to mean exchange for something.

So:

• обмен на другой размер = exchange for another size

This is a standard pattern:

• обмен на другой товар = exchange for another item
• обмен на больший размер = exchange for a larger size

Here другой размер is in the accusative case after на. Since размер is masculine and inanimate, the accusative looks the same as the nominative:

• nominative: другой размер
• accusative: другой размер

It is just the normal Russian expression. Languages divide these ideas differently.

In English, you might say:

• exchange it for another size

In Russian, the natural phrasing is:

• обмен на другой размер

So this is something to learn as a fixed collocation rather than translate word-for-word.

Оказался means turned out to be or proved to be.

So:

• обмен ... оказался невозможен = the exchange turned out to be impossible

This suggests that maybe an exchange was considered or attempted, but in the end it was not possible.

Compare:

• был невозможен = was impossible
• оказался невозможен = turned out to be impossible

The version with оказался feels more natural when describing the result of checking the situation.

Невозможен is the short form of the adjective невозможный.

In Russian, short-form adjectives are often used in predicate position, especially after verbs like:

• быть = to be
• стать = to become
• оказаться = to turn out to be

So:

• обмен оказался невозможен = the exchange turned out to be impossible

Here невозможен agrees with обмен, which is masculine singular.

Forms would be:

• masculine: невозможен
• feminine: невозможна
• neuter: невозможно
• plural: невозможны

Невозможным would be instrumental, which is not used here.

Yes. Russian word order is flexible, although some versions sound more natural in certain contexts.

Original:

• Сестра оформила возврат, потому что обмен на другой размер оказался невозможен.

You could also say:

• Потому что обмен на другой размер оказался невозможен, сестра оформила возврат.

That puts more focus on the reason first.

Another very natural alternative would be:

• Обмен на другой размер оказался невозможен, поэтому сестра оформила возврат.
• The exchange for another size turned out to be impossible, so the sister processed a return.

So the meaning stays similar, but the emphasis shifts.

Not necessarily. Сестра just means sister.

Russian often leaves out possessives like my, his, or her when the context already makes it clear.

Depending on context, сестра could mean:

• my sister
• his sister
• her sister
• the sister

If the speaker wanted to be explicit, they could say:

• моя сестра = my sister

But Russian often sounds more natural without the possessive when it is obvious.

Because the whole sentence describes a completed situation in the past:

• оформила = she processed / arranged
• оказался = it turned out

Also, both verbs are perfective:

• оформить → оформила
• оказаться → оказался

This gives a sense of completed events:

1. it turned out that the exchange was impossible
2. as a result, she processed the return

So the sentence presents the situation as a finished sequence of events.

A very natural English equivalent is:

• She processed the return because an exchange for another size turned out to be impossible.

Or more idiomatically:

• She arranged a return because it turned out that exchanging it for another size wasn’t possible.

This helps show how Russian packs the idea into a concise, formal-sounding structure centered on nouns like возврат and обмен.