Мы ушли, поскольку было поздно.

Ушли is the past tense plural form of the verb уйти (to leave, to go away).

It breaks down like this:

• уйти = infinitive
• ушёл = past masculine singular
• ушла = past feminine singular
• ушло = past neuter singular
• ушли = past plural

Since the subject is мы (we), the verb must be plural, so Russian uses ушли.

This is about aspect, which is very important in Russian.

• уйти → ушли = perfective
• уходить → уходили = imperfective

In this sentence, ушли shows a completed action: we left.
The idea is that the leaving happened and was finished.

If you said Мы уходили, it would usually suggest something more like:

• we were leaving
• we used to leave
• we left / were in the process of leaving (depending on context)

So Мы ушли, поскольку было поздно means that the group actually left, as a completed event, because it was late.

Поскольку means since or because.

It introduces a reason:

• Мы ушли, поскольку было поздно.
• We left, since/because it was late.

It is perfectly correct, but it often sounds a bit more formal, written, or bookish than потому что.

Very roughly:

• поскольку = since / because (often slightly more formal)
• потому что = because (very common in everyday speech)
• так как = since / as (also common, a bit more formal than потому что)

So a learner should recognize that поскольку is normal Russian, but not always the most conversational choice.

Yes. That would be very natural:

• Мы ушли, потому что было поздно.

This is probably the version many learners will hear more often in everyday conversation.

The meaning stays basically the same. The main difference is style:

• потому что = more neutral and conversational
• поскольку = often a little more formal or literary

Because поскольку introduces a subordinate clause.

The sentence has two parts:

• Мы ушли = main clause
• поскольку было поздно = subordinate clause giving the reason

In Russian, subordinate clauses are normally separated by a comma, so:

• Мы ушли, поскольку было поздно.

If the order is reversed, the comma is still needed:

• Поскольку было поздно, мы ушли.

Because this is an impersonal construction.

Russian often uses было in the neuter singular when there is no real grammatical subject, especially in expressions about:

• time
• weather
• physical or emotional states
• general conditions

Here, было поздно means it was late.

There is no actual Russian word corresponding to English it here. Russian simply uses the default impersonal form:

• было холодно = it was cold
• было темно = it was dark
• было поздно = it was late

So было is not agreeing with мы. It belongs to a separate impersonal clause.

Here поздно is not an adjective like late in a late train.
It is functioning as a predicative word (often called a category of state word), meaning something like it is late.

Compare:

• поздний поезд = a late train
  
  • here поздний is an adjective
• было поздно = it was late
  
  • here поздно is a predicative/adverb-like form

This is why you do not say:

• было поздний ❌

Instead you say:

• было поздно ✅

Russian often does not use a dummy subject like English it.

English says:

• It was late.
• It was cold.
• It is raining.

Russian often just uses an impersonal verb form:

• Было поздно.
• Было холодно.
• Идёт дождь. / Дождь идёт.

So in было поздно, there is no separate word for it because Russian simply does not need one in this structure.

Yes, absolutely:

• Поскольку было поздно, мы ушли.

This means the same thing: Since it was late, we left.

The difference is mainly one of focus and flow:

• Мы ушли, поскольку было поздно.
  starts with the action, then gives the reason
• Поскольку было поздно, мы ушли.
  starts with the reason, then gives the result

Both are correct.

Russian often omits subject pronouns when they are clear from context, but in the past tense the verb does not show person clearly, only gender/number.

Compare:

• ушли = left, but it could mean they left or we left depending on context

So мы is useful here because it clearly tells you who left.

If the context already made it obvious, Russian could omit it:

• Ушли, поскольку было поздно.

But standing alone, that sounds less clear than the full sentence.

It can mean both, depending on context.

Уйти / ушли often carries the idea of:

• leave
• go away
• depart
• walk off

So Мы ушли can be understood as:

• We left
• We went away

In this sentence, the natural English translation is usually We left, because it was late.

Usually, no—not for the same meaning.

• Было поздно = it was late
• Нам было поздно would suggest something more like it was late for us or we were late, depending on context, and that is a different idea

If you want to say that the time was late in general, Russian normally says:

• было поздно

So the sentence is about the general situation being late, not about lateness to us specifically.

Yes, it is grammatical and natural. But in everyday speech, many speakers would more often say:

• Мы ушли, потому что было поздно.

Your original sentence with поскольку is still correct and natural, especially in:

• written Russian
• careful speech
• formal or explanatory contexts

So a learner should understand it well, even if they may hear потому что more often in conversation.