Он работает в одиночку.

Работает is the 3rd person singular present-tense form of работать (to work).

It matches он (he), so:

• я работаю — I work
• ты работаешь — you work
• он работает — he works

So the basic structure is:

Он + работает = He works / He is working

It can mean either one.

Russian present tense does not make the same strict distinction English makes between:

• He works alone
• He is working alone

Both can be expressed by Он работает в одиночку. The exact meaning depends on context.

Without extra context, it often sounds like a general fact or usual way of working: He works alone.

В одиночку is an adverbial expression meaning alone, on one’s own, or single-handedly.

It often emphasizes that someone is doing something without partners, teammates, or help.

Compare:

• Он работает один — He is working alone / He is alone while working
• Он работает в одиночку — He works on his own, without others involved

So в одиночку usually feels a bit stronger and more specific than just один.

Yes, you can.

But the nuance is slightly different:

• Он работает один can simply describe the situation: he is alone.
• Он работает в одиночку stresses that he does the work by himself, without assistance, not as part of a group.

In many contexts both are possible, but в одиночку is often the better choice when you want to emphasize solo work rather than just physical aloneness.

Because в одиночку is a fixed Russian expression.

You should learn it as a set phrase meaning:

• alone
• on one’s own
• single-handedly

Even though в often means in or into, here the whole phrase functions like an adverb in English. So it is best not to translate it word-for-word, but to remember в одиночку as one unit.

Одиночку is in the accusative singular.

The base noun is одиночка. In the fixed phrase в одиночку, it appears in the accusative form.

For most learners, the important thing is this:

• memorize в одиночку as a complete expression
• do not try to change it freely

So yes, there is a case explanation behind it, but in practice this is mainly an idiomatic phrase you learn as a whole.

Sometimes, but they are not exact matches.

• сам often means himself and can emphasize he, not someone else
• самостоятельно means independently, without supervision, or on one’s own

Compare:

• Он работает сам — He himself is working
• Он работает самостоятельно — He works independently
• Он работает в одиночку — He works alone / single-handedly

So в одиночку is the best choice when the main idea is without other people.

Russian uses the imperfective here because the sentence describes:

• an ongoing activity,
• a general habit,
• or a process.

Работать is the normal verb for to work in this kind of statement.

A perfective verb would not fit the same way, because perfective verbs usually point to a completed result or a more limited event. Here the idea is simply that he works in that manner.

Yes, if the context already makes it clear who is being talked about.

For example, in conversation you might hear:

Работает в одиночку.

That can mean He works alone if the subject is understood from the previous sentence.

Still, when the sentence stands by itself, Он работает в одиночку is the most neutral and complete version.

No. Russian word order is fairly flexible.

The neutral order is:

Он работает в одиночку.

But you can move things around for emphasis:

• В одиночку он работает. — emphasizes alone
• Работает он в одиночку. — can sound contrastive or explanatory

So the original sentence is the most natural basic version, but other orders are possible depending on what you want to stress.

The stress is:

Он рабо́тает в одино́чку.

Main stressed syllables:

• рабо́тает
• одино́чку

If you want a rough pronunciation guide:

on ra-BO-ta-yet v a-dee-NOCH-ku

The exact vowel sounds change a bit in unstressed syllables, but the most important thing for learners is to place the stress correctly.