Когда поезд остановился, я быстро вошёл в вагон.

Когда means when. Here it introduces a subordinate clause: Когда поезд остановился = When the train stopped.

Russian often uses когда exactly like English when in time clauses:

• Когда я пришёл, он уже спал. = When I arrived, he was already asleep.

Because the когда-clause comes first, it is followed by a comma before the main clause.

Russian uses a comma to separate the subordinate clause from the main clause.

So:

• Когда поезд остановился, = subordinate clause
• я быстро вошёл в вагон. = main clause

This is standard Russian punctuation. Even though English also usually uses a comma when when... comes first, in Russian this separation is especially regular and important.

Остановился is the perfective past form of остановиться, which means to stop in the sense of reaching a completed result.

Here the sentence describes a completed event:

• first the train stopped
• then I got into the carriage

So perfective is natural.

Compare:

• поезд остановился = the train stopped / came to a stop
• поезд останавливался = the train used to stop / was stopping / stopped on various occasions, depending on context

In this sentence, we need the single completed action, so остановился is the right choice.

The verb is остановиться. The -ся part is a reflexive marker, but in many verbs it does not translate directly as oneself.

Here:

• остановить = to stop something
• остановиться = to stop, to come to a stop

So:

• Машинист остановил поезд. = The driver stopped the train.
• Поезд остановился. = The train stopped.

In other words, -ся often helps turn a transitive verb into one that means the subject undergoes the action.

Вошёл is the past tense of войти, a perfective verb meaning to go in / enter.

It focuses on the completed action of entering:

• я вошёл в вагон = I entered the carriage / I got into the carriage

Compare:

• шёл = was going / walked; it describes movement in progress, not the completed entry
• заходил = was going in / used to go in / dropped in, depending on context; usually not the best choice here
• входил = was entering / used to enter
• вошёл = entered, completed

Since the speaker actually got in, Russian uses the perfective вошёл.

Russian uses в + accusative for movement into something enclosed.

So:

• в вагон = into the carriage
• в дом = into the house
• в комнату = into the room

На is used for movement onto a surface or with some forms of transport and public places:

• на стол = onto the table
• на поезд can mean onto the train in the sense of boarding the train as a whole

But в вагон is very natural when focusing on entering the actual carriage. It emphasizes going inside the physical car.

It is in the accusative case.

The preposition в can take different cases:

• в + accusative = motion into
• в + prepositional = location inside

Compare:

• Я вошёл в вагон. = I entered the carriage.
• Я сидел в вагоне. = I was sitting in the carriage.

So:

• в вагон = motion, accusative
• в вагоне = location, prepositional

This is just the masculine singular past tense form of войти.

Russian past tense agrees with gender and number:

• я вошёл = I entered (male speaker)
• я вошла = I entered (female speaker)
• мы вошли = we entered

So вошёл tells you the speaker is male, or at least the sentence is written from a male perspective.

Because поезд is the subject of the clause поезд остановился.

The subject of a sentence is normally in the nominative:

• поезд = the train
• поезд остановился = the train stopped

Nothing is acting on the train here; the train is the thing performing or undergoing the action in the sentence structure, so nominative is used.

Yes, often it can be omitted if the subject is already clear from context.

So both are possible:

• Когда поезд остановился, я быстро вошёл в вагон.
• Когда поезд остановился, быстро вошёл в вагон.

However, Russian includes я when the speaker wants to be clearer, more explicit, or slightly more natural in a standalone sentence. Since this sentence is presented by itself, keeping я sounds completely normal.

Быстро means quickly and functions as an adverb.

In Russian, adverb placement is flexible, but быстро вошёл is a very natural neutral order:

• я быстро вошёл в вагон

Other orders are possible for emphasis:

• я вошёл в вагон быстро
• я вошёл быстро в вагон

But the given version sounds the most standard and smooth in ordinary speech.

No. Вагон means a carriage, car, or wagon — one part of a train.

• поезд = train
• вагон = carriage / car

So the sentence literally describes:

• the train stopped
• I entered a carriage

In English, people often just say I got on the train, but Russian can be more specific about entering the carriage: вошёл в вагон.

Yes. Even though когда literally means when, in context the sequence is clear:

1. the train stopped
2. then I entered the carriage

So in natural English, depending on context, it could be translated as:

• When the train stopped, I quickly entered the carriage
• Once the train had stopped, I quickly got into the carriage
• After the train stopped, I quickly got into the carriage

The Russian sentence itself simply states the timing; the exact English wording can vary.