Ночью форточка была закрыта, но в комнате всё равно было прохладно.

Ночью is the instrumental case of ночь.

In Russian, some time expressions use the instrumental without a preposition to mean during a certain part of the day or time period. So:

• утром = in the morning
• днём = during the day / in the daytime
• вечером = in the evening
• ночью = at night / during the night

So Ночью форточка была закрыта means At night, the vent window was closed.

This is a very common pattern, and it is best to learn these as fixed time expressions.

Форточка is a very specific kind of window: a small opening window, often a little top or side section of a larger window used for ventilation.

It is not just any window. A full window is окно, but форточка is the little part you open to let air in.

For many English speakers, there is no perfect everyday one-word translation, so you may see it translated as:

• vent window
• small window pane
• little window flap
• sometimes just window if the context is clear

Была закрыта means was closed and describes the state of the vent window.

Breakdown:

• была = was (past tense of быть)
• закрыта = short-form passive participle / predicate adjective meaning closed

So форточка была закрыта = the vent window was closed.

Why not закрылась?

• закрылась means closed itself or got closed
• it focuses more on the action or change
• here, the sentence is describing the condition at that time, not the process

Why not закрытая?

• закрытая is the full form
• full forms are usually used like normal adjectives before a noun: закрытая форточка = a closed vent window
• after быть, Russian normally uses the short form in this kind of predicate: форточка была закрыта

Because it agrees with форточка, which is:

• feminine
• singular
• nominative

So the short form must also be feminine singular:

• masculine: закрыт
• feminine: закрыта
• neuter: закрыто
• plural: закрыты

Since форточка is feminine, we get:

• форточка была закрыта

Because в with the prepositional case is used to mean in when talking about location.

• комната = room
• prepositional singular = в комнате = in the room

Compare:

• в комнате = in the room, inside the room, location
• в комнату = into the room, direction

Here the sentence talks about where it was cool, so location is needed:

• в комнате всё равно было прохладно = it was still cool in the room

Here всё равно means:

• all the same
• nevertheless
• still
• anyway

In this sentence it shows that the second fact happened despite the first one:

• Ночью форточка была закрыта, но в комнате всё равно было прохладно.
• The vent window was closed at night, but it was still cool in the room.

A very useful thing to know is that всё равно has more than one use in Russian. It can also mean:

• I don’t care / it makes no difference to me

For example:

• Мне всё равно. = I don’t care / It’s all the same to me.

But in your sentence, it clearly means still / nevertheless.

Because прохладно is not a normal adjective here. It is a predicative adverb / category-of-state word, used in an impersonal construction.

Russian often describes weather, temperature, atmosphere, and general conditions with this pattern:

• было холодно = it was cold
• было жарко = it was hot
• было темно = it was dark
• было тихо = it was quiet
• было прохладно = it was cool

There is no subject like it in Russian, so the verb usually appears in neuter singular:

• было

So:

• в комнате было прохладно = it was cool in the room

If you used прохладная, that would need a feminine noun, for example:

• комната была прохладная = the room was cool

That is possible, but it is a different structure and slightly less neutral than the impersonal в комнате было прохладно.

Because the two parts of the sentence are built differently.

1. форточка была закрыта

   • This clause has a subject: форточка
   • форточка is feminine singular
   • so the verb is была

2. в комнате всё равно было прохладно

   • This is an impersonal clause
   • there is no real grammatical subject
   • Russian uses neuter singular in the past for this kind of statement
   • so the verb is было

So the different forms are completely normal:

• была because of a feminine subject
• было because the second clause is impersonal

Но means but and marks a clear contrast:

• the vent window was closed
• but the room was still cool

This is a natural choice because the second part goes against what you might expect from the first.

Could а be used? Sometimes Russian uses а for contrast too, but но is stronger for unexpected contradiction. In this sentence, но is the more natural choice because the idea is:

• even though the window vent was closed, the room was still cool

So но fits the logic very well.

Not exactly.

Прохладно usually means:

• cool
• a bit chilly
• not warm

It is milder than холодно.

Compare:

• прохладно = cool, pleasantly or mildly cool
• холодно = cold

So in this sentence, прохладно suggests the room was noticeably cool, but not necessarily freezing or unpleasantly cold.

The word order is natural, but Russian word order is fairly flexible.

The given sentence:

• Ночью форточка была закрыта, но в комнате всё равно было прохладно.

is a normal, neutral way to say it.

You could move things around for emphasis. For example:

• Форточка ночью была закрыта, но в комнате всё равно было прохладно.
• Но в комнате всё равно было прохладно, хотя ночью форточка была закрыта.

These are possible, but they shift emphasis or style a bit.

The original sentence sounds very natural because:

• Ночью sets the time first
• then comes the first fact
• then но introduces the surprising result

Not in standard past-time narration.

Because the sentence clearly refers to the past situation at night, Russian normally uses the past tense:

• форточка была закрыта

If you say:

• форточка закрыта

that usually means the vent window is closed now, in the present.

So in your sentence, была is needed to match the past-time setting.

Yes, historically it is the same word, but in всё равно the whole expression functions as a set phrase.

On its own:

• всё can mean everything or all

But in всё равно, you should usually understand the phrase as a unit meaning:

• all the same
• nevertheless
• still
• it makes no difference

So it is better to learn всё равно as one chunk rather than trying to translate each word separately every time.