Сядь подальше от окна.

Сядь is the informal singular imperative of сесть, a perfective verb meaning to sit down / to take a seat.

So:

• сесть = to sit down
• сядь = sit down! / take a seat! said to one person informally

It is not the ordinary present tense form. It is specifically a command.

Both can be used, but they are not exactly the same.

• сядь comes from сесть (perfective)
• садись comes from садиться (imperfective)

A useful practical difference is:

• сядь focuses on the result: take a seat, get yourself seated
• садись often sounds a bit more like go ahead and sit down, and can feel more conversational or less abrupt in many situations

In this sentence, сядь sounds like a direct instruction to sit in a different place.
If you said Садись подальше от окна, that would also be possible, but the nuance is slightly different.

Yes. Сядь is for:

• one person
• informal address

For:

• more than one person, or
• formal/polite address to one person

you use сядьте:

• Сядьте подальше от окна.

So the sentence you were given is the informal singular version.

Подальше means farther away or a bit farther away.

It comes from дальше (farther / further), with the prefix по-, which often gives a sense like:

• a little farther
• somewhat farther
• or simply a natural Russian way to say farther away in commands like this

In everyday Russian, подальше is very common in phrases telling someone to move away from something:

• отойди подальше = move farther away
• сядь подальше = sit farther away

Yes, дальше от окна is possible, but подальше от окна is often more natural in this kind of instruction.

A rough comparison:

• дальше от окна = farther from the window
• подальше от окна = a bit farther from the window / farther away from the window

In real speech, подальше is very idiomatic after commands, especially when telling someone to keep more distance from something.

Because the preposition от requires the genitive case.

The noun is:

• окно = window

Its genitive singular form is:

• окна

So:

• от окна = from the window / away from the window

This is a very common pattern in Russian:

• от дома = from the house
• от двери = from the door
• от окна = from the window

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

In English, commands often have an understood you:

• Sit farther from the window.

Russian works the same way here. The imperative сядь already tells you that the subject is you.

You could add ты, but it would usually sound emphatic, contrastive, or unnatural unless there is a reason:

• Ты сядь подальше от окна.

That might sound like you sit farther from the window in contrast to someone else.

Yes, Russian word order is flexible, but different orders change the focus or emphasis.

The given order is very natural for a simple command:

• Сядь подальше от окна.

It starts with the action, then gives the location.

Other orders are possible, for example:

• Подальше от окна сядь.
• Сядь от окна подальше.

These are understandable, but they sound more marked or conversational, with emphasis falling differently.
For a learner, the original order is the safest and most natural one to use.

Literally, it tells someone to sit farther from the window.

If the person is not seated yet, it naturally means:

• choose a seat farther from the window

If the person is already sitting and you want them to change seats, Russian often prefers a different verb:

• пересядь подальше от окна = move to another seat farther from the window

So сядь is about the act of sitting down, while пересядь is specifically move and sit somewhere else.

Yes, especially for a learner.

Stress:

• Сядь — one syllable, so it is automatically stressed
• пода́льше — stress on а
• окна́ — stress on the final а

So the sentence is pronounced roughly like:

• Сядь пода́льше от окна́.

A couple of pronunciation notes:

• сядь has a soft с and a soft final дь
• от is short and unstressed
• in natural speech, подальше от flows together quite smoothly

Russian has no articles. There is no direct equivalent of a or the.

So окна by itself can mean:

• a window
• the window

depending on context.

In this sentence, English would usually say the window, but Russian simply uses the noun without an article:

• от окна

The context tells you which window is meant.

Not necessarily, but it is definitely a direct command.

Whether it sounds rude depends on:

• tone of voice
• relationship between speakers
• situation

For example, it could sound normal if:

• a parent is speaking to a child
• a teacher is organizing seating
• someone is concerned about a draft or cold air

If you want to sound softer, Russian often uses gentler wording, for example:

• Сядь, пожалуйста, подальше от окна.
• Садись, пожалуйста, подальше от окна.

Adding пожалуйста makes it much more polite.