Мы остались наедине.

Because the subject is мы = we.

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

• я остался / осталась
• ты остался / осталась
• он остался
• она осталась
• мы остались
• вы остались
• они остались

So остались is the correct past plural form for мы.

The dictionary form is остаться.

So:

• остаться = perfective infinitive
• остались = past tense, plural

A very rough meaning of остаться is to remain, to stay, to be left, or to end up staying, depending on context.

Because остаться does not just describe a state; it often describes the idea of remaining or ending up in that state.

So Мы остались наедине is closer to:

• We were left alone
• We ended up alone together
• We remained alone together

If you used были, as in Мы были наедине, that would simply describe the situation as a fact:

• We were alone together / in private

So the difference is roughly:

• были наедине = description of a state
• остались наедине = we came to be in that state, or were left in that state

Наедине means alone together, in private, or one-on-one, depending on context.

It usually implies that someone is with one other person or with a limited person/group but without others around, especially in the sense of privacy.

So Мы остались наедине usually means that we were left by ourselves, with no one else present.

This is a very common expression in Russian.

Because the two expressions are similar, but not identical.

• одни means alone in the sense of by ourselves
• наедине often emphasizes privacy or being alone together

Compare:

• Мы остались одни. = We were left alone.
• Мы остались наедине. = We were left alone together / in private.

In many situations they are close in meaning, but наедине more strongly suggests a private situation, especially between two people.

It is written as one word: наедине.

This can be confusing because it begins with на-, which often looks like the preposition на. But here, learners should treat наедине as a fixed adverbial expression.

So write:

• Мы остались наедине.

not:

• Мы остались на едине. ❌

Not in a way that is useful to analyze for everyday learning.

Although наедине historically comes from an older structure, in modern Russian it is best learned as a fixed adverb meaning in private / alone together.

So for a learner, the simplest approach is:

• memorize наедине as a whole word/expression
• do not try to build it productively the way you would with ordinary на + noun

Остаться is perfective.

Its imperfective partner is оставаться.

Here the perfective is used because the sentence presents the situation as a completed result:

• there was some previous situation
• then others left, or circumstances changed
• as a result, we ended up alone together

Compare:

• Мы оставались наедине. = We were staying/remained alone together, perhaps over some period of time or repeatedly in context.
• Мы остались наедине. = We ended up alone together / were left alone together.

Usually наедине most naturally suggests two people in private, especially in everyday conversation.

With мы, it often means the two of us were left alone.

In some broader contexts it can refer more generally to being left without outsiders present, but the strongest and most common sense is private one-on-one company.

So if a learner sees Мы остались наедине, the most natural interpretation is:

• The two of us were left alone together.

The basic meaning stays the same, but the emphasis can shift.

Neutral word order:

• Мы остались наедине.

Other possible orders:

• Наедине мы остались.
• Остались мы наедине.

These alternatives are more marked and may sound literary, emotional, or context-dependent.

For most learners, Мы остались наедине is the normal, neutral version to use.

You would change the verb to match the speaker’s gender and number.

Examples:

• Я остался наедине. = I was left alone / in private. (male speaker)
• Я осталась наедине. = I was left alone / in private. (female speaker)

But note that with я, наедине can sometimes mean alone with someone/something, depending on context, not always literally completely by myself.

For example:

• Я остался наедине с собой. = I was left alone with myself.
• Она осталась наедине с ним. = She was left alone with him.

Yes, very often.

A common pattern is:

• наедине с + instrumental

Examples:

• Я остался наедине с ней. = I was left alone with her.
• Она хотела поговорить с ним наедине. = She wanted to speak with him in private.
• Он остался наедине со своими мыслями. = He was left alone with his thoughts.

So наедине often appears either:

• by itself, when the people involved are already clear
• or with с + instrumental, when Russian explicitly says who someone is alone with