Я сел в поезд и сразу открыл книгу.

Сел is the past tense of the perfective verb сесть.

Here it shows a completed action: first the person got onto / sat down in the train, then did the next action.

Compare:

• сел — completed action
• садился — was getting on / used to get on / the process of getting on
• сидел — was sitting

In a short story-like sentence with one action after another, Russian very often uses perfective past:

• Я сел в поезд и сразу открыл книгу.

That gives a clear sequence: finished action 1 -> finished action 2.

Literally, сесть means to sit down, but with transport it often means to get on / board.

So сел в поезд is understood naturally as got into the train / boarded the train.

Russian often uses verbs of sitting for boarding transport:

• сесть в машину
• сесть в автобус
• сесть в поезд

So yes, the basic verb is about sitting, but in this context it has the practical meaning of boarding.

Because Russian uses different cases after в depending on the meaning:

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

So:

• в поезд — into the train
• в поезде — in the train

Here the person is moving into the train, so Russian uses accusative:

• Я сел в поезд

If you wanted to say someone was already there, you would use:

• Я сидел в поезде — I was sitting in the train

Yes, sometimes на поезд is also possible, but the nuance can differ a little.

A simple way to think about it:

• в поезд emphasizes entering the train
• на поезд can emphasize taking/catching the train as transport

So in this sentence, в поезд fits well because the next action happens once the person is inside the train.

Learners should know that Russian can use both в and на with transport in different contexts, but в поезд here is perfectly natural.

For the same reason as сел: it is a perfective past tense form, showing a completed action.

The sentence tells a sequence of events:

1. сел
2. открыл

So открыл means the person opened the book as a completed event.

Compare:

• открыл — opened, completed
• открывал — was opening / used to open / opened repeatedly / background action

In Russian narration, a chain of finished actions is usually expressed with perfective verbs.

Because книгу is the accusative singular of книга.

It is the direct object of открыл:

• открыть что? — книгу

For a feminine noun ending in -а, the accusative singular often changes -а -> -у:

• книга -> книгу
• газета -> газету
• машина -> машину

So:

• открыл книгу = opened a/the book

In the past tense, Russian verbs agree with the subject in gender and number.

For a male speaker:

• я сел
• я открыл

For a female speaker:

• я села
• я открыла

For plural:

• мы сели
• мы открыли

So this sentence suggests that the speaker is male, unless it is just being used as a dictionary/example form.

Yes. Russian often omits subject pronouns when the subject is already clear from context.

So both are possible:

• Я сел в поезд и сразу открыл книгу.
• Сел в поезд и сразу открыл книгу.

The version with я sounds a little more explicit. It may be used:

• for clarity
• at the start of a story
• for slight emphasis on I

So я is not required, but it is completely normal.

Сразу means immediately / right away, and here it modifies открыл.

The placement:

• и сразу открыл книгу

is a very natural, neutral word order in Russian.

It highlights that the second action happened immediately after the first one.

Russian word order is flexible, so other versions are possible, but they may shift emphasis:

• Я сел в поезд и сразу открыл книгу. — neutral
• Я сел в поезд и открыл книгу сразу. — more marked
• Сразу я сел в поезд... — emphasizes immediately in a different way

The original word order is the most straightforward.

Because Russian has no articles.

Russian does not have separate words equivalent to English a/an and the. Whether something is a train, the train, a book, or the book is understood from:

• context
• word order
• previous mention
• the overall situation

So поезд and книгу can correspond to either a or the in English, depending on context.