Когда поезд остановится, выйди на платформу.

This is a very common difference between English and Russian.

In Russian, after когда referring to the future, you normally use a real future form:

• Когда поезд остановится, выйди на платформу.
• literally: When the train will stop, get out onto the platform.

English avoids will after when, but Russian does not. So когда + future is completely normal here.

Also, остановится is perfective future, which fits the meaning well: first the train comes to a stop, then you get out.

Because the sentence is about a single future completed action.

• остановится = will stop / will come to a stop
• останавливается = stops / is stopping / usually stops

Here the speaker means one specific future event: the train will stop, and after that you should act. That is why Russian uses the perfective future остановится.

If you said Когда поезд останавливается..., it would sound more like a general/habitual statement, such as When the train stops (as a rule / in general)...

This is about aspect in the imperative.

• выйди = perfective imperative
• выходи = imperfective imperative

In this sentence, the speaker wants one complete action: step out / get out once the train has stopped. That makes выйди the natural choice.

Very roughly:

• выйди = get out, step out, do it
• выходи = be getting out, start getting out, or sometimes get out regularly / in this general situation

So Когда поезд остановится, выйди на платформу means: once that event happens, perform the complete action.

Because на can take different cases depending on whether you mean movement toward something or location on something.

Here:

• на платформу = onto the platform → accusative, because there is movement
• на платформе = on the platform → prepositional, because it describes location

Compare:

• Выйди на платформу. = Go out onto the platform.
• Жди на платформе. = Wait on the platform.

So the accusative платформу is required because the person is moving onto the platform.

Выйти is a very common verb meaning to go out, to come out, or to get out, depending on context.

In transport contexts, it often means:

• get out
• get off
• exit

So in this sentence, выйди на платформу is something like:

• get out onto the platform
• step out onto the platform

The exact English translation depends on context, but the Russian verb is very natural here.

Yes, остановится comes from остановиться, which has -ся.

Historically and grammatically, this is called a reflexive form, but in many verbs it does not translate as a literal reflexive like stop oneself. Instead, it is simply the normal Russian verb for to stop in this sense.

Compare:

• остановить поезд = to stop the train
  someone stops something
• поезд остановится = the train will stop
  the train stops

So -ся here helps form the intransitive meaning: the subject undergoes the action rather than actively stopping something else.

Because Когда поезд остановится is a subordinate clause.

Russian normally separates subordinate clauses with commas, including clauses introduced by когда.

So:

• Когда поезд остановится, выйди на платформу.

The comma is required.

Even if English punctuation may feel lighter in some similar sentences, Russian is stricter about this.

Because Russian often leaves out subject pronouns when they are clear from the verb form.

Here выйди is an imperative addressed to one person informally, so ты is understood automatically.

So the sentence effectively means:

• Когда поезд остановится, ты выйди на платформу.

But adding ты is usually unnecessary unless you want emphasis or contrast.

For formal or plural you, the verb would be:

• выйдите

So:

• Когда поезд остановится, выйдите на платформу.

Поезд is in the nominative case because it is the subject of the clause:

• поезд остановится = the train will stop

The train is the thing performing the action, so nominative is exactly what you would expect here.

It is possible, but it changes the nuance.

• выйди focuses on one completed action
• выходи can sound more like go ahead and get out then, start getting out then, or sometimes a less bounded instruction

In this specific sentence, выйди is the cleaner and more standard choice because the idea is:

1. the train stops
2. then perform one complete action: get out onto the platform

So выйди is the best fit. Выходи is not the most neutral choice here.

Yes, to some extent. Russian word order is fairly flexible, although not random.

The neutral order here is:

• Когда поезд остановится, выйди на платформу.

You could also rearrange parts for emphasis, but the original is the most natural standard phrasing.

For example, putting на платформу earlier could emphasize the destination, but in everyday instructions the given version sounds normal and clear.

So for a learner, this sentence is a good model to remember as it is.