Мы расстались вчера.

It comes from the verb расстаться, which means to part, to separate, or in some contexts to break up.

So:

• расстаться = infinitive
• расстались = we parted / we broke up / we separated

This verb is very commonly used both for:

• people physically going different ways
• ending a relationship

The exact meaning depends on context.

Because it is:

• past tense
• plural
• reflexive

Let’s break it down:

• -л- = past tense marker
• -и = plural ending
• -сь = reflexive ending

So расстались literally has the structure parted-themselves in a grammatical sense, though in natural English you would just say we parted or we broke up.

Russian does not use an auxiliary verb like English did or were to form the simple past.

English:

• We parted yesterday
• We broke up yesterday

Russian:

• Мы расстались вчера.

The past meaning is built directly into the verb form расстались.

Yes, -сь is the reflexive marker, a shorter form of -ся.

In Russian, many verbs use -ся / -сь, but they are not always literally reflexive in the English sense. With расстаться, the reflexive form is simply the normal dictionary form of the verb.

So although historically or grammatically it is reflexive, you should learn расстаться as a complete verb meaning:

• to part
• to separate
• to break up

You do not normally say расстать without the reflexive ending in this meaning.

Because мы means we, and Russian past tense agrees with the subject in gender/number.

Past tense forms of this verb would be:

• я расстался = I parted / I broke up (male speaker)
• я рассталась = I parted / I broke up (female speaker)
• мы расстались = we parted / we broke up
• они расстались = they parted / they broke up

Since мы is plural, the verb must be plural too: расстались.

Расстаться is perfective.

That means it presents the action as a completed event. In this sentence, the idea is that the separation happened as a finished event yesterday.

So Мы расстались вчера means something like:

• We parted yesterday
• We broke up yesterday

The imperfective partner is usually расставаться, which would be used for repeated action, process, or background context, for example:

• Мы часто расставались и мирились. = We often broke up and made up.

In your sentence, the perfective makes sense because it refers to one completed event.

Вчера means yesterday, and here it is an adverb, not a noun in a case form.

So it does not change for case in this sentence.

It simply tells you when the action happened:

• Мы расстались вчера. = We parted yesterday.

Russian often uses time adverbs like this very directly:

• сегодня = today
• вчера = yesterday
• завтра = tomorrow

Yes. Russian word order is more flexible than English.

These are all possible, though they can sound slightly different in emphasis:

• Мы расстались вчера.
• Вчера мы расстались.
• Мы вчера расстались.

Basic differences:

• Мы расстались вчера. = neutral, straightforward
• Вчера мы расстались. = stronger emphasis on yesterday
• Мы вчера расстались. = also natural; often conversational

So the meaning stays basically the same, but the focus can shift.

Yes, Russian can sometimes omit the subject pronoun if it is clear from context.

So in conversation, you might hear:

• Расстались вчера.

This can mean We broke up yesterday or We parted yesterday, if the context already makes the subject obvious.

However, Мы расстались вчера is completely normal and clear. Including мы can make the sentence more explicit or contrastive.

Yes. Расстались can cover more than one situation.

Depending on context, it could mean:

• we said goodbye / parted ways
• we separated
• we broke up as a couple

For example:

• After a meeting: Мы расстались вчера вечером.
  = We parted yesterday evening.

• Talking about a relationship: Мы расстались вчера.
  = We broke up yesterday.

So context is very important for the exact English translation.

The stress is on the second а:

расста́лись

And the infinitive is:

расста́ться

A rough pronunciation guide:

• мы ≈ mɨ
• расста́лись ≈ ras-STA-lis'
• вчера́ ≈ fchee-RA

Stress matters in Russian, so it is worth learning the stressed forms together with the word.

Putting вчера at the end is a normal neutral choice in Russian. It often sounds like simple topic + action + time:

• Мы = subject
• расстались = action
• вчера = time

But Russian does not require the time expression to go at the end. It can move depending on emphasis:

• Вчера мы расстались.
• Мы вчера расстались.
• Мы расстались вчера.

So the end position here is natural, but not the only correct option.