Мы ушли, ибо было поздно.

Ушли is the past tense plural form of уйти, which means to leave / to go away.

• уйти = perfective verb
• ушёл / ушла / ушло / ушли = past tense forms

Because the subject is мы (we), the verb takes the plural form ушли.

The perfective verb is used because the action is presented as completed: we left.

This is because уйти has an irregular-looking past stem.

• infinitive: уйти
• past stem: уш-
• past plural: ушли

Many Russian verbs change their stem in the past tense, so this is something learners usually just memorize.

Compare:

• прийти → пришёл / пришли
• найти → нашёл / нашли
• уйти → ушёл / ушли

So ушли is the normal past plural form of уйти, even though it does not closely resemble the infinitive.

Ибо means because, for, or since.

In modern everyday Russian, ибо sounds bookish, formal, or sometimes old-fashioned. In normal conversation, people usually say:

• Мы ушли, потому что было поздно.
• Мы ушли, так как было поздно.

So this sentence is grammatically correct, but ибо is not the most colloquial choice.

In Russian, a comma is normally used before conjunctions that introduce a subordinate clause, including ибо.

So:

• Мы ушли, ибо было поздно.

The comma separates:

1. the main clause: Мы ушли
2. the reason clause: ибо было поздно

This is similar to how Russian also uses commas with:

• потому что
• так как
• когда
• если

Поздно here is an adverb/predicative word meaning late in the sense of it was late.

Russian often uses this kind of structure for states or situations:

• было поздно = it was late
• было холодно = it was cold
• было трудно = it was difficult

This is not an adjective describing a noun. There is no noun like вечер or час in the sentence. Instead, the sentence describes the general situation.

That is why поздно is used, not an adjective like поздний.

Because было поздно is an impersonal construction.

In Russian, expressions like it was late, it was cold, it was necessary, it was possible often use the past tense form было. This does not agree with мы.

The structure is basically:

• было поздно = it was late

There is no real subject in that clause. So было is the normal form.

Compare:

• Было темно. = It was dark.
• Было жарко. = It was hot.
• Было поздно. = It was late.

Yes. Russian often omits subject pronouns when they are clear from the verb form.

So all of these are possible:

• Мы ушли, ибо было поздно.
• Ушли, ибо было поздно.

The version with мы is a little more explicit and can add emphasis or clarity. But in many contexts, ушли already tells you the subject is we.

No, they are related but different.

• уйти / ушли = to leave / go away
• пойти / пошли = to set off / start going

So:

• Мы ушли = We left
• Мы пошли = We went / started going

In this sentence, ушли fits better because the idea is that we left due to it being late.

Yes. The most natural everyday version would be:

• Мы ушли, потому что было поздно.

This means the same thing. The difference is mainly style:

• ибо = literary, formal, somewhat archaic
• потому что = neutral and very common

So if you are speaking casually, потому что is usually the safer choice.

They belong to different parts of speech.

• поздно = late as a state or adverb
  
  • Было поздно. = It was late.
• поздний = late as an adjective describing a noun
  
  • поздний вечер = late evening
  • поздний поезд = late train

In your sentence, Russian needs the state expression было поздно, not an adjective.

Yes, Russian word order is fairly flexible, although each version may sound slightly different in emphasis.

Standard order:

• Мы ушли, ибо было поздно.

Possible variation:

• Ибо было поздно, мы ушли.

That second version sounds more literary or stylistically marked. The original order is more straightforward.

So yes, word order can change, but the given version is the most neutral for this style.

Not always. It can introduce a reason clause after the main clause, as here:

• Мы ушли, ибо было поздно.

It can also appear at the beginning in more literary language:

• Ибо было поздно, мы ушли.

However, this fronted use is less common in ordinary modern speech and sounds more formal or rhetorical.

A more natural conversational version would usually be:

• Мы ушли, потому что было поздно.

Or even:

• Мы ушли — было уже поздно.
• Мы ушли, так как было поздно.

So the original sentence is correct, but ибо makes it sound more written, literary, or elevated than everyday speech.