Не сиди у окна: от форточки идёт сквозняк.

Because не сиди is the negative imperative: it means don’t sit / don’t be sitting.

• сидеть = to sit
• сиди = sit / be sitting (imperative, singular, informal)
• не сиди = don’t sit / don’t stay sitting

Russian usually forms this kind of command with не + imperative form, not with the infinitive.

This is an aspect/meaning difference:

• сидеть = to be sitting
• садиться = to sit down

So:

• Не сиди у окна = Don’t sit / don’t stay by the window
• Не садись у окна = Don’t sit down by the window

The sentence is warning someone about being in that location, not just about the moment of taking a seat there, so не сиди fits well.

Because the preposition у takes the genitive case when it means by, near, at.

• окно = window
• genitive singular = окна
• therefore: у окна = by the window / near the window

So the form is determined by the preposition.

Here у means by, near, or next to.

So у окна is not in the window and not on the window. It means the person is located close to it.

Compare:

• у окна = by the window
• в окне = in the window
• на окне = on the window / on the windowsill area

Here it is genitive singular, not plural.

That can be confusing because the spelling matches another form:

• окна́ = genitive singular of окно
• о́кна = nominative plural, windows

In this sentence, у requires the genitive, so it must be у окна́ = by the window.

The stress helps distinguish them in speech.

Форточка is a small window vent or small opening part of a window, often a little hinged pane used for ventilation.

It is a very common word in Russian, especially for the kind of small opening in older-style windows. It is not usually the whole window, but a smaller part you open to let air in.

So от форточки suggests the draught is coming from that little open vent.

Because от here means from in the sense of coming from the direction/source of something.

• от форточки = from the vent / from the area of the vent
• из форточки would sound more literally like out of the opening

With things like cold air, smell, sound, and draughts, Russian often uses от + genitive to mark the source:

• от двери дует = it’s blowing from the door
• от окна идёт холод = cold is coming from the window

So от форточки sounds natural here.

This is a very natural Russian way to speak.

The verb идти literally means to go, but Russian uses it much more broadly than English. It can describe something happening, moving, flowing, or coming from somewhere.

So идёт сквозняк means something like:

• there’s a draught
• a draught is coming through
• a draught is coming from there

It is idiomatic Russian, just like:

• идёт дождь = it’s raining
• идёт снег = it’s snowing

So you should not translate идёт too literally here.

Сквозняк is not just any breeze. It means a draught / draft, especially a current of air moving through a room because air is passing between openings, such as a door and a window or a vent.

It often has the idea of an unpleasant or unhealthy cold current of air.

This is also culturally important: in Russian-speaking contexts, сквозняк is very often treated as something you should avoid.

The colon shows that the second part gives the reason for the first part.

• Не сиди у окна = Don’t sit by the window.
• от форточки идёт сквозняк = there’s a draught coming from the vent.

So the second clause explains why the speaker says not to sit there.

In English, you could also express this with:

• Don’t sit by the window: there’s a draught coming from the vent.
• Don’t sit by the window, because there’s a draught coming from the vent.

It is informal singular.

The form сиди is used when speaking to:

• one person
• in an informal way

If you were speaking politely or to more than one person, you would use:

• Не сидите у окна

So the sentence as written sounds like something said to one person the speaker addresses with ты.

Yes, Russian word order is flexible, though the original sounds very natural.

For example, you could also hear:

• Не сиди у окна: сквозняк идёт от форточки.

That still means basically the same thing. The original version puts a little more focus on the source first:

• от форточки идёт сквозняк = it’s from the vent that the draught is coming

So the chosen order sounds natural and slightly emphasizes where the problem is coming from.

Russian often does not use a present-tense equivalent of there is / there are the way English does.

In this sentence, Russian simply says:

• идёт сквозняк

That is enough to express the idea that a draught is present / happening.

So instead of building the sentence around there is, Russian uses a normal verb phrase. That is very common.