Свет проходит сквозь окно.

Проходит is the 3rd person singular present-tense form of проходить.

• проходить = to pass, to go through
• проходит = passes / is passing / goes through

It is singular because the subject свет is singular.

Russian often uses the present tense for a general fact or an ongoing process, so this sentence can describe something like light regularly or currently passing through the window.

A perfective verb would not be used here in the same way. For example, пройдёт would mean will pass through and would refer to a completed future event, not a general present situation.

Свет is the subject of the sentence, so it is in the nominative case.

In this sentence:

• свет = the thing doing the action
• проходит = the action
• сквозь окно = where/how it passes

So the structure is basically:

• Light
  • passes
    • through the window

Also, Russian has no articles, so свет can mean light, the light, or sometimes some light, depending on context.

Yes, it is completely normal.

Here проходить means something like:

• to pass through
• to go through

With light, Russian commonly uses verbs of movement this way. So свет проходит сквозь окно is a natural way to say that light is coming through or passing through the window.

English often prefers comes through, while Russian can naturally say passes through.

Both сквозь and через can mean through, but they are not always identical.

Сквозь often emphasizes passing through something, especially when the thing is:

• an opening
• a layer
• a barrier
• something semi-transparent or penetrable

It is very common with things like:

• light
• sound
• wind
• fog
• glass
• cracks
• windows

So сквозь окно sounds very natural here.

Через окно is also possible in some contexts, but it can sound more neutral, more like by way of the window or through the window as a route/opening. With light, сквозь often feels more vivid and fitting.

After сквозь, Russian uses the accusative case.

So окно here is in the accusative singular.

The reason it looks unchanged is that окно is a neuter noun, and for many neuter nouns, the nominative singular and accusative singular forms are the same:

• nominative: окно
• accusative: окно

So the case is different grammatically, even though the form does not change.

Compare with a feminine noun, where you can see the change more clearly:

• сквозь дверь
• сквозь стену

Yes. Russian word order is flexible.

The neutral order here is:

• Свет проходит сквозь окно.

But you could also say:

• Сквозь окно проходит свет.

This version puts more emphasis on сквозь окно, as if setting the scene first: Through the window, light passes.

So the basic meaning stays the same, but the focus or style shifts a little.

The stress is:

• Свет
• прохо́дит
• сквозь
• окно́

So the full sentence is:

Свет прохо́дит сквозь окно́.

A rough pronunciation guide:

• свет ≈ svyet
• прохо́дит ≈ pra-KHO-deet
• сквозь ≈ skvozʹ
• окно́ ≈ ak-NO

A learner should especially notice:

• unstressed о is reduced in pronunciation
• сквозь ends with a soft sign, so the final consonant is softened

The verb проходить is imperfective, and that fits the meaning because the sentence describes:

• an ongoing process
• a general fact
• a repeated or natural phenomenon

Light passing through a window is not a single completed event here; it is just what is happening or what generally happens.

If you used a perfective verb, the meaning would change to a single completed event, usually in the future. For example:

• Свет пройдёт сквозь окно. = The light will pass through the window.

That is a different idea from the original sentence.