Они остались вдвоём.

Остались is the past tense plural form of остаться.

• остаться = to remain / to stay / to be left
• остались = they remained / they stayed / they were left

Because the subject is они (they), the verb is in the plural past form.

Because the subject is они (they). In Russian, past tense verbs agree with the subject in number, and in the singular they also agree in gender.

Compare:

• он остался = he remained
• она осталась = she remained
• они остались = they remained

So here the -ли shows plural.

Because остаться is a reflexive verb, and -ся / -сь is part of that verb.

This does not literally mean themselves here. It is simply part of the normal verb form.

A useful comparison:

• оставить = to leave something / someone
• остаться = to remain / stay behind / be left

So остались is the normal past plural form of остаться.

Остались is perfective, while оставались is imperfective.

In this sentence, Russian uses остались because it presents the situation as a result:

• Они остались вдвоём. = They ended up alone together / They were left as just the two of them.

If you said оставались, it would sound more like a process, duration, or repeated situation:

• Они оставались вдвоём весь вечер. = They stayed alone together all evening.

So остались fits a single completed outcome.

Вдвоём means the two of them together, as a pair, or just the two of them.

It often corresponds to English phrases like:

• together as two
• just the two of them
• alone together (depending on context)

So it focuses on the fact that there are two people together and no others in the group with them.

Because вдвоём has a different job in the sentence.

• два = the basic number two
• двое = a collective numeral, often used for two people
• вдвоём = an adverb meaning as two / the two together

In this sentence, Russian is not simply counting them. It is describing how they remained: as two, together.

So:

• их было двое = there were two of them
• они остались вдвоём = they remained just the two of them

You would not normally say они остались два.

Yes. Вдвоём is used like an adverb.

It answers something like:

• how?
• in what grouping?
• how many together?

It does not change for case, number, or gender.

That is why the form stays the same:

• мы вдвоём
• они вдвоём
• остались вдвоём

Not exactly. Literally, it means the two together.

Very often, this implies no one else was there with them, so English may naturally translate it as:

• alone together
• just the two of them

But the core idea is still two people together, not the emotional idea of isolation.

So вдвоём emphasizes number and grouping first.

Yes. The exact English wording depends on context.

Because остаться can mean to remain, to stay, or to be left, this sentence can suggest:

• They remained just the two of them
• They were left alone together
• They stayed behind together, the two of them

Russian often leaves that nuance to context.

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

For example:

• Они остались вдвоём. = neutral
• Вдвоём они остались. = stronger emphasis on just the two of them
• Остались они вдвоём. = more marked or stylistic

The original sentence is the most neutral everyday order.

Because this is a plural past tense form.

In Russian past tense:

• singular forms show gender:
  • остался = masculine
  • осталась = feminine
  • осталось = neuter
• plural does not show gender:
  • остались

So with они, you only see plural, not masculine/feminine distinction.

It is pronounced roughly vdva-YOM.

The ё is important because it shows both:

• the correct vowel sound
• the stress

So the stress is on the last syllable: вдвоём.

In many Russian texts, native speakers write е instead of ё, so you may see вдвоем, but the intended pronunciation is still вдвоём.