Когда автобус останавливается у вокзала, я выхожу.

Because the sentence describes a habitual / repeated action:

• Когда автобус останавливается у вокзала, я выхожу.
• Literally: When/Whenever the bus stops by the station, I get off.

In Russian, the present tense is very commonly used for things that happen regularly or as a general rule. English often does the same:

• When the bus stops, I get off.

So this is not necessarily about right now. It usually means this is what I do whenever that happens.

This is an aspect question.

• останавливаться / останавливаться = imperfective
• остановиться = perfective

In the sentence, останавливается is the imperfective present, which fits a repeated, ongoing, or habitual situation.

So:

• Когда автобус останавливается у вокзала, я выхожу.
  = Whenever the bus stops by the station, I get off.

If you used остановится, that would be perfective future:

• Когда автобус остановится у вокзала, я выйду.
  = When the bus stops by the station, I will get off.

That version refers more naturally to one future event, not a repeated habit.

For the same reason: aspect.

• выходить = imperfective
• выйти = perfective

выхожу is imperfective present, which works well for a habitual action:

• Когда автобус останавливается у вокзала, я выхожу.
• Whenever the bus stops at the station, I get off.

If you say:

• Когда автобус остановится у вокзала, я выйду.

that means:

• When the bus stops at the station, I will get off.

So:

• выхожу = what I generally do
• выйду = what I will do once, in the future

It can be understood as either, but here it is most naturally whenever because the whole sentence is habitual.

• Когда автобус останавливается у вокзала, я выхожу.

This means something like:

• When the bus stops at the station, I get off
• Whenever the bus stops at the station, I get off

Russian когда covers both meanings, and the exact sense comes from context.

Because Когда автобус останавливается у вокзала is a subordinate clause, and Russian separates subordinate clauses with a comma.

Structure:

• Когда автобус останавливается у вокзала, = subordinate clause
• я выхожу. = main clause

Russian punctuation is stricter than English here. With когда, a comma is normally required.

Because the preposition у requires the genitive case.

• dictionary form: вокзал
• genitive singular: вокзала

So:

• у вокзала = by the station / near the station

This is a very common pattern:

• у дома = by the house
• у метро = by the metro
• у школы = by the school

So the ending -а on вокзала shows the genitive singular.

У вокзала means by the station, near the station, or sometimes loosely at the station.

The preposition у usually means next to / by / near.

So the image is that the bus stops by the station building or at the station area, and then the speaker gets off.

Depending on context, English might translate it as:

• by the station
• near the station
• at the station

Sometimes, but it is not exactly the same.

• у вокзала = by / near the station
• на вокзале = at the station / on the station premises

In this sentence, у вокзала sounds natural if the bus stops outside or by the station.

If you say на вокзале, it suggests being at the station area itself. In some contexts that may also work, but the nuance is different.

So у вокзала is a good, natural choice for the bus stops by the station.

Here is the grammar:

останавливается

This comes from останавливаться / останавливаться = to stop.

Form:

• автобус = he/it (masculine singular)
• so the verb is 3rd person singular
• -ся is the reflexive ending

So останавливается means:

• stops
• literally something like is stopping

выхожу

This comes from выходить = to go out, get out, get off

Form:

• я = I
• so this is 1st person singular

So я выхожу = I get out / I get off / I go out

In transport contexts, выхожу often means I get off.

Because автобус is the subject of the clause:

• автобус останавливается = the bus stops

Subjects are normally in the nominative case in Russian.

So:

• автобус = nominative singular
• вокзала = genitive singular after у
• я = nominative singular subject of выхожу

Yes. Russian word order is relatively flexible.

The original sentence:

• Когда автобус останавливается у вокзала, я выхожу.

You could also say:

• Я выхожу, когда автобус останавливается у вокзала.

Both are grammatical and mean essentially the same thing.

The difference is mostly emphasis:

• starting with Когда... focuses first on the condition/time
• starting with Я выхожу... focuses first on what I do

Russian often uses word order for nuance rather than basic grammar.

Yes, literally выходить means to go out / to exit.

But in context, Russian often uses выходить for leaving a vehicle:

• выхожу из автобуса = I get off the bus / I get out of the bus

When the vehicle is obvious from context, Russian may simply say:

• я выхожу = I get off

So the translation depends on context, not just the dictionary meaning of the verb.

Most naturally, it describes a general routine.

That is because:

• both verbs are in the present tense
• both are imperfective
• the structure with когда sounds habitual here

So it usually means:

• Whenever the bus stops by the station, I get off

If you wanted a single future event, Russian would more naturally use perfective future verbs:

• Когда автобус остановится у вокзала, я выйду.

Russian has no articles like a / an / the.

So:

• автобус can mean a bus or the bus
• вокзал can mean a station or the station

The exact meaning comes from context.

In this sentence, English naturally uses the because the speaker likely means a specific known stop or station:

• When the bus stops at the station, I get off.

But Russian does not need a separate word for that.