Когда урок закончится, я пойду домой пешком.

Russian works differently here.

After когда, Russian normally uses a real future form if the action is future:

• Когда урок закончится, я пойду домой.

So this literally looks like When the lesson will end, I will go home, but that is exactly how Russian expresses it.

A very important point:

• закончится is the future of the perfective verb закончиться
• perfective future often means will be completed / will come to an end

So in Russian, this is the normal way to say when the lesson is over / when the lesson ends.

Because the sentence refers to one completed event in the future.

• закончится = will end / will be over → one future completion
• заканчивается = ends / is ending → present or habitual meaning

Compare:

• Когда урок закончится, я пойду домой.
  = When the lesson ends/is over, I’ll go home.

• Когда урок заканчивается, я обычно иду домой.
  = When the lesson ends, I usually go home.
  This sounds like a repeated/habitual situation.

So закончится is used because this sentence is about a specific future occasion.

In this sentence, закончиться means to end / to come to an end.

Compare:

• закончить что-то = to finish something

  • Я закончил урок. = I finished the lesson.

• закончиться = to end

  • Урок закончился. = The lesson ended.

So the -ся version is used when the subject is the thing that ends:

• урок закончится = the lesson will end

It is not literally reflexive in English. You usually just translate it as end.

Пойду is the normal choice here because it means I will set off / I will go as a single future action.

Russian often uses a perfective verb for a one-time future action:

• пойти → пойду = I will go / I’ll head off

Compare:

• я пойду домой = I’ll go home
  This is the natural choice here.

• я буду идти домой = I will be walking home
  This emphasizes the process, like I’ll be in the middle of walking home. It does not fit as well here.

• я иду домой = I am going home / I’m on my way home
  Present tense, not right for this future sentence.

So пойду is best because the speaker means once the lesson ends, I’ll leave and go home.

They are related, but the aspect is different:

• идти = imperfective, to be going / to go on foot
• пойти = perfective, to start going / to go off

In future contexts, Russian often prefers the perfective verb when talking about a single completed or definite action:

• я пойду домой = I’ll go home
• я буду идти домой = I’ll be walking home

So пойду is not just future of идти in a simple mechanical sense; it is the perfective partner that gives the natural meaning I’ll set off and go home.

Домой is a special word meaning homeward / to home / home as a direction.

• домой = movement toward home
• дома = at home
• дом = house/home
• к дому = toward the house/building

Examples:

• Я иду домой. = I’m going home.
• Я дома. = I’m at home.
• Я подхожу к дому. = I’m approaching the house.

So in your sentence:

• я пойду домой = I’ll go home

This is the normal Russian way to say it.

Пешком means on foot.

Russian uses this word by itself, without a preposition:

• идти пешком = to go on foot
• пойти домой пешком = to go home on foot

This is just the standard expression. English uses on foot, but Russian does not need an equivalent of on here.

Historically, пешком is related to the instrumental case, but for a learner it is best to remember it as a fixed adverbial word meaning on foot.

Yes. Russian word order is flexible.

All of these are possible:

• Когда урок закончится, я пойду домой пешком.
• Я пойду домой пешком, когда урок закончится.
• Когда урок закончится, домой я пойду пешком.
  This adds emphasis.

The most neutral version is the original:

• Когда урок закончится, я пойду домой пешком.

Russian changes word order mainly for:

• emphasis
• topic/focus
• style

But the basic meaning stays the same.

Because Когда урок закончится is a subordinate clause.

Russian uses commas to separate subordinate clauses from the main clause:

• Когда урок закончится, я пойду домой пешком.

This is similar to English:

• When the lesson ends, I’ll go home on foot.

If the subordinate clause comes second, there is still a comma:

• Я пойду домой пешком, когда урок закончится.

So the comma is required by normal Russian punctuation.

Yes, often you could.

Russian often drops subject pronouns when the verb ending already makes the subject clear:

• Когда урок закончится, пойду домой пешком.

This still clearly means I’ll go home on foot because пойду already shows I.

However, keeping я is also perfectly normal. It can make the sentence:

• slightly clearer
• slightly more explicit
• a bit more contrastive, depending on context

So both are possible:

• Когда урок закончится, я пойду домой пешком.
• Когда урок закончится, пойду домой пешком.

Урок is in the nominative case because it is the subject of закончится.

The structure is:

• урок закончится = the lesson will end

Here, урок is the thing doing the action grammatically, so nominative is correct.

If you used the transitive verb закончить, then you would need an object:

• Я закончу урок.
  Here урок is in the accusative, but for masculine inanimate nouns like урок, accusative looks the same as nominative.

So in your sentence, think of урок simply as the subject: the lesson.

It is mainly when the lesson ends / when the lesson is over.

In practice, it often implies that the second action happens after that point:

• first the lesson ends
• then I go home

So in English, depending on context, you might translate it as:

• When the lesson ends, I’ll go home on foot.
• Once the lesson is over, I’ll walk home.
• After the lesson ends, I’ll go home on foot.

But the Russian conjunction here is specifically когда = when.

You could, but it would change the focus and sound less natural in this sentence.

• Я пойду домой пешком = I’ll go home on foot
  Neutral, natural, and best here.

• Я буду идти домой пешком = I’ll be walking home
  This emphasizes the ongoing process.

You would use буду идти more naturally in a context like:

• В семь часов я буду идти домой.
  = At seven o’clock I’ll be walking home.

So for a simple future plan after the lesson ends, пойду is the better choice.

It goes with the whole idea of motion, and its position is somewhat flexible.

Common placements:

• Я пойду домой пешком.
• Я пешком пойду домой.
• Домой я пойду пешком.

The most neutral version is the original one, with пешком near the end.

If you move it earlier, you usually add emphasis:

• Я пешком пойду домой, а не поеду.
  = I’ll go home on foot, not by vehicle.

So yes, it can move, but the original order is the most natural and least marked.