Она пришла в одиночку.

Because the subject она is feminine, and in the past tense Russian verbs agree with gender and number.

• он пришел = he came
• она пришла = she came
• оно пришло = it came
• они пришли = they came

So пришла is the correct feminine singular past form.

The dictionary form is прийти.

This is a very common verb meaning to come / to arrive.
Its past-tense forms are irregular enough that learners often need to memorize them:

• прийти
• пришел / пришла / пришло / пришли

So она пришла is the past-tense feminine form of прийти.

Russian does not use auxiliary verbs like English did, has, or was to form the simple past in sentences like this.

English says:

• She came
• She has come

Russian simply uses the past form of the main verb:

• Она пришла

The exact English translation depends on context, but the Russian sentence itself does not need an extra helper verb.

Прийти is a perfective verb, and пришла presents the action as a completed event: she arrived.

That fits this sentence well, because it refers to one completed act of coming.

The imperfective partner is usually приходить:

• Она приходила в одиночку.

That would usually mean something more like:

• she used to come alone
• she came alone (at some point, with focus on the process or fact rather than the completed arrival)

So:

• Она пришла в одиночку. = one completed arrival
• Она приходила в одиночку. = habitual, repeated, or less result-focused

В одиночку is a fixed adverbial expression. It functions like an adverb and describes how she came.

It means something like:

• alone
• by herself
• on her own

So grammatically, it is not the main action; it modifies the verb пришла.

You can think of the structure as:

• Она = subject
• пришла = verb
• в одиночку = adverbial phrase describing the manner

Both are possible, but they are not exactly identical in feel.

• Она пришла одна.
• Она пришла в одиночку.

Одна is the simpler and more neutral way to say alone.

В одиночку often sounds a bit more expressive and can suggest:

• unaccompanied
• on her own
• without help or company

In many contexts, the difference is small, but в одиночку can feel slightly more emphatic or stylistically marked than одна.

It is in the accusative case.

The expression в одиночку is a fixed idiomatic phrase, and after в it uses the accusative here. Learners usually do best to memorize it as a whole expression rather than trying to build it from scratch each time.

So it is best learned as:

• в одиночку = alone / on one’s own

rather than separately as в + noun with a fully productive pattern.

No. В одиночку stays the same.

For example:

• Он пришел в одиночку.
• Она пришла в одиночку.
• Они пришли в одиночку.

The verb changes to match the subject, but в одиночку does not.

Yes. Russian word order is flexible, and changing it usually changes emphasis rather than basic meaning.

For example:

• Она пришла в одиночку. = neutral
• В одиночку она пришла. = stronger emphasis on the fact that she came alone
• Пришла она в одиночку. = more marked, often used in storytelling or contrast

The most neutral version is the original:

• Она пришла в одиночку.

Yes. In English, came and arrived can both work depending on context.

Russian пришла focuses on the completed coming/arrival, so in many situations English might naturally translate it as either:

• She came alone
• She arrived alone

The best English choice depends on the wider context, not on a different Russian form in this sentence.

Historically and in the system of Russian motion verbs, идти / прийти are connected with going on foot. Because of that, learners often notice this point.

In modern everyday Russian, though, она пришла is very commonly used simply for she came / she arrived, even when the exact mode of transport is not important.

If Russian specifically wants to emphasize transport by vehicle, other verbs may be used, for example:

• Она приехала = she came/arrived by vehicle

But in many ordinary contexts, пришла is perfectly natural as a general came.

The stress is:

• она́ пришла́ в одино́чку

A few pronunciation notes:

• пришла́ has stress on the last syllable.
• The шл cluster in пришла can feel difficult at first; try saying it smoothly as one group: при-шла.
• одино́чку has stress on но́.

So the sentence is pronounced approximately:

• ана ПРИШЛА в адиНОчку
  (very rough approximation for English speakers)

But it is best to imitate native audio if possible.