Как бы поздно ни было, мама сначала проверяет, закрыто ли окно возле кроватки.

This is a fixed concessive pattern meaning no matter how late it is / however late it may be.

The structure is:

• как бы + adverb/adjective + ни + verb

So here:

• как бы поздно ни было = however late it was / no matter how late it was

In natural English for this sentence, you would usually translate it as no matter how late it is.

This pattern is very common in Russian for ideas like however, no matter how, whatever, etc.

Examples:

• Как бы трудно ни было, он продолжал работать. = No matter how hard it was, he kept working.
• Что бы ни случилось, позвони мне. = Whatever happens, call me.

In this kind of concessive expression, Russian uses ни, not не.

Compare:

• не = ordinary negation
• ни = often used in fixed patterns like whatever / however / no matter...

So in:

• Как бы поздно ни было

the ни does not mean simple negation. It helps form the meaning no matter how late.

This is a pattern you mostly learn as a whole expression:

• кто бы ни... = whoever
• что бы ни... = whatever
• где бы ни... = wherever
• как бы ни... = however / no matter how

This is a great question, because it looks strange at first.

In the phrase как бы поздно ни было, Russian uses было in a fixed idiomatic pattern. Even when the main clause is in the present, this concessive expression often keeps было.

So the whole thing means:

• No matter how late it is, Mom first checks...

Even though было is formally past tense, the expression is often best understood more generally, not as a real past-time statement.

This is one of those places where Russian grammar and English grammar do not line up neatly word-for-word.

Because поздно here is not an adjective modifying a noun. It is a predicative adverb/state word, meaning late in the sense of it is late.

Compare:

• поздний час = a late hour
• поздняя ночь = a late night
• уже поздно = it is already late

In this sentence, the meaning is however late it is, so Russian uses поздно.

Because this is an impersonal construction.

Russian often uses neuter singular verb forms in sentences where English uses it is:

• Было поздно. = It was late.
• Темнеет. = It is getting dark.

There is no real grammatical subject here. So the verb appears in the default neuter singular form:

• было

Сначала means first, at first, or first of all, depending on context.

Here it means:

• Mom first checks whether the window...

It suggests this is the first thing she does before doing something else.

Compare:

• Сначала поужинаем, потом пойдём гулять. = First we'll have dinner, then we'll go for a walk.

Because the sentence describes a repeated/habitual action.

• проверяет = she checks / she is in the habit of checking

Russian uses the imperfective for:

• repeated actions
• regular habits
• general facts

If you used проверит, that would mean she will check once in the future.

So here:

• мама сначала проверяет... = Mom first checks...

This sounds like something she regularly does.

Закрыто ли окно means whether the window is closed.

The particle ли is used to form an indirect yes/no question.

Compare:

Direct question:

• Окно закрыто? = Is the window closed?

Indirect question:

• Она проверяет, закрыто ли окно. = She checks whether the window is closed.

A useful way to think about ли:

• it often corresponds to English whether

In indirect yes/no questions, ли usually comes after the word that is being focused first in the clause.

So:

• закрыто ли окно literally feels like closed-whether the window

Russian often places ли after the predicate or the element under question.

This word order is normal and natural:

• Я не знаю, придёт ли он. = I don't know whether he will come.
• Она спросила, дома ли мама. = She asked whether Mom was home.
• Он проверил, закрыта ли дверь. = He checked whether the door was closed.

Because окно is neuter singular, and закрыто agrees with it.

• окно = neuter singular
• so the short form is закрыто

Compare:

• дверь закрыта = the door is closed
• окно закрыто = the window is closed
• двери закрыты = the doors are closed

Also, this is the short form, which is commonly used in predicates expressing a state:

• Окно закрыто. = The window is closed.

Not:

• закрытый — that is the full form and usually works more like an attributive adjective:
  • закрытое окно = a closed window

For a learner, the most practical answer is: it behaves like a short-form predicate word meaning a resulting state.

In sentences like:

• Окно закрыто
• Дверь открыта

Russian uses short forms that are often taught as short-form adjectives or short passive participles, depending on the word and the analysis.

What matters most here is:

• it means is closed
• it agrees with окно
• it is used predicatively, not before the noun

So:

• закрыто ли окно = whether the window is closed

Because возле requires the genitive case.

Dictionary form:

• кроватка = little bed / crib / cot

Genitive singular:

• кроватки

So:

• возле кроватки = near the crib / by the little bed

Other examples:

• возле дома = near the house
• возле двери = near the door

Кроватка is a diminutive form of кровать.

• кровать = bed
• кроватка = little bed

In many contexts, especially with a child, кроватка often means:

• crib
• cot
• little bed

So окно возле кроватки suggests a window near a child’s bed or crib.

There are two subordinate clauses, so commas are needed.

The sentence is:

• Как бы поздно ни было, мама сначала проверяет, закрыто ли окно возле кроватки.

Comma 1:

• after было
• this separates the concessive clause:
  • Как бы поздно ни было = No matter how late it is

Comma 2:

• before закрыто ли окно...
• this introduces the indirect question:
  • закрыто ли окно возле кроватки = whether the window near the crib is closed

So the structure is:

1. concessive clause
2. main clause
3. indirect question clause

Not very naturally.

A very literal version would be something like:

• However late it may be, Mom first checks whether the window near the crib is closed.

But natural English would usually be:

• No matter how late it is, Mom first checks whether the window by the crib is closed.

Russian often uses structures that do not match English word-for-word, especially with:

• как бы... ни...
• ли clauses
• impersonal expressions like поздно

So it is better to understand the grammar pattern than to translate each word mechanically.