Я открою форточку, чтобы в комнате стал воздух посвежее.

Форточка is a small window panel or vent window, often a little section of a larger window that you open for air. It is more specific than окно (window). In older Russian homes and apartments, this is a very common word.

So открыть форточку means not just open the window, but more specifically open the little vent window.

Открою is the 1st person singular future of the perfective verb открыть.

Russian uses:

• perfective future for a single completed action: Я открою = I will open
• imperfective for process, repetition, or ongoing action: Я открываю / буду открывать

Here the speaker means one concrete action with a result: I’ll open it. That is why открою fits best.

Because it is the direct object of открою.

The verb открыть takes the accusative case, and форточка is a feminine noun, so:

• nominative: форточка
• accusative: форточку

So:

• Я открою что? → форточку

Here чтобы means so that or in order that.

It introduces the purpose or intended result of the main action:

• Я открою форточку = the action
• чтобы в комнате стал воздух посвежее = the purpose/result intended

So the structure is basically:

• I’ll open the vent window so that...

Because чтобы introduces a subordinate clause.

Russian normally separates this kind of clause with a comma:

• Я открою форточку, чтобы...

This is standard punctuation.

This is a very common question.

After чтобы, Russian often uses a verb form that looks like the past tense:

• чтобы воздух стал...

But here it does not mean past time. It expresses something wanted, intended, or possible after the main action. In English, it often translates as:

• so that the air becomes...
• so that the air will become...
• so that the air would become...

So стал here is normal after чтобы and does not mean that the air already became fresher in the past.

Because the subject is воздух, and воздух is masculine singular.

So the verb agrees with it:

• воздух стал

If the subject were neuter, you would get стало. If it were feminine, you would get стала.

Russian word order is much freer than English word order.

Both are possible:

• чтобы в комнате воздух стал посвежее
• чтобы в комнате стал воздух посвежее

The version in your sentence sounds natural and a bit more fluid/colloquial. Putting воздух after стал can make the result feel a little more like new information being introduced.

So this is mostly about word order and emphasis, not a change in basic meaning.

В комнате is prepositional case after в, because it expresses location:

• в комнате = in the room

Compare:

• в комнате = in the room, inside the room already
• в комнату = into the room, motion toward it

Here the meaning is location, so в комнате is correct.

Посвежее means a bit fresher or somewhat fresher.

It comes from the comparative свежее (fresher) with the prefix по-, which often softens the meaning:

• свежее = fresher
• посвежее = a little fresher / somewhat fresher

This по- often makes the comparison sound less strong, more natural, or more conversational.

You see the same idea in words like:

• побольше = a bit bigger / more
• потише = a bit quieter
• получше = a bit better

Yes.

• чтобы воздух стал свежее = so that the air became fresher
• чтобы воздух стал посвежее = so that the air became a bit fresher

Both are correct. Посвежее sounds slightly softer and more idiomatic in everyday speech.

You could say that, but the meaning shifts a little.

• чтобы в комнате стал воздух посвежее focuses on change: the air should become fresher
• чтобы в комнате был свежий воздух focuses more on the resulting state: there should be fresh air in the room

Because opening a vent window causes a change in the air, стал is very natural here.

Yes, but it would express the idea a little differently.

For example:

• Я открою форточку, чтобы проветрить комнату.

That means:

• I’ll open the vent window to air out the room.

This version focuses on the speaker’s action/purpose.

Your original sentence:

• Я открою форточку, чтобы в комнате стал воздух посвежее.

focuses more on the desired result: the air in the room becoming fresher.

So both are correct, but they emphasize different things.