Мы говорим «едва я вошёл, как...» или «едва поезд остановился, как...», когда второе действие начинается почти сразу.

Here едва is functioning as part of a time-linking construction, not just as the adverb barely.

In едва X, как Y, it introduces the first event and signals that the second event happens almost immediately after it.

A good English equivalent is:

• hardly had X happened when Y happened
• no sooner had X happened than Y happened
• sometimes simply as soon as X happened

So in this structure, think of едва as a trigger for the idea of almost no time gap.

No. In this sentence, как does not mean how.

It is a conjunction that forms the second half of the pattern едва... как....

So:

• едва я вошёл, как он заговорил
  = hardly had I entered when he started talking

Here как is closer to when or part of an English pattern like no sooner... than..., not how.

You can omit как.

Both are possible:

• Едва я вошёл, как он заговорил.
• Едва я вошёл, он заговорил.

With как, the link between the two actions sounds more explicit and a bit more set or literary. Without как, the sentence is still perfectly normal.

So for a learner:

• едва..., как... = very standard pattern
• едва..., ... = also correct

Because there are two clauses here.

• едва я вошёл = first clause
• как он заговорил = second clause

Russian normally separates these clauses with a comma.

So yes, the comma is required in standard writing:

• Едва я вошёл, как он заговорил.

And if как is omitted, you still keep the comma:

• Едва я вошёл, он заговорил.

Because this construction usually presents the first action as a completed event after which the second action begins almost immediately.

• вошёл = entered / finished entering
• остановился = stopped / came to a stop

That completed first action creates the point from which the next action starts.

This is why perfective is very natural here: it marks a single finished event.

Imperfective can appear in some habitual or repeated situations, but the default in examples like this is perfective.

Because Russian past tense in the singular shows gender.

With я, the form depends on whether the speaker is male or female:

• male speaker: я вошёл
• female speaker: я вошла

In the example, вошёл implies a male speaker.

The same idea appears in поезд остановился:

• поезд is a masculine noun
• so the verb is masculine too: остановился

Yes, Russian word order is flexible, but some orders are more natural than others.

The most straightforward pattern is:

• Едва я вошёл, как он заговорил.

You may also see more marked or literary orders such as:

• Едва вошёл я, как он заговорил.

But for learning purposes, it is best to stick with:

• едва + clause, как + clause

Also, как normally stays at the start of the second clause in this construction.

No. It can also be used in the future and in habitual present situations.

Examples:

• Едва он придёт, как мы начнём.
  = As soon as he arrives, we’ll start.

• Едва он входит, как все замолкают.
  = Whenever he comes in, everyone falls silent.

That said, this pattern is especially common in narrative past situations, which is why learners often first meet it with past-tense verbs.

Both can often be translated as as soon as, but they are not exactly identical in feel.

Как только is usually the more neutral, everyday option:

• Как только поезд остановился, мы вышли.

Едва... как... often emphasizes that the second action followed with almost no delay at all:

• Едва поезд остановился, как мы вышли.

So едва... как... can sound a bit closer to:

• hardly had the train stopped when we got out
• no sooner had the train stopped than we got out

It is often slightly more vivid or literary.

It is not strange or incorrect in normal Russian, but it often sounds a little more literary or narrative than the most everyday alternatives.

You will commonly meet it in:

• written Russian
• storytelling
• more careful speech

In casual everyday conversation, many speakers may more often choose как только.

So the construction is absolutely useful to know, but its style is often a bit less plain than the simplest conversational option.

Because the sentence is teaching a pattern, not giving a full example.

So:

• едва я вошёл, как...
• едва поезд остановился, как...

mean: this is the frame; now add any suitable second action after it.

For example:

• Едва я вошёл, как все замолчали.
• Едва поезд остановился, как пассажиры начали выходить.

The ... just shows that the author is illustrating the structure itself.