Новая грамматическая тема: конструкция «не успел ... как ...» показывает, что второе действие произошло почти сразу.

Успел is the masculine singular past-tense form of the perfective verb успеть.

In grammar explanations, Russian often shows a pattern in a kind of default form, usually masculine singular past. It does not mean you must always say успел.

In real sentences, the form changes to match the subject:

• Я не успел войти, как все замолчали.
• Она не успела войти, как все замолчали.
• Мы не успели войти, как все замолчали.

So the construction is really a model, and успел is just the citation form used in the explanation.

Literally, it is close to:

• He/she/I hadn’t managed to ... when/suddenly ...
• or more naturally in English: No sooner had ... than ...

The idea is that the second action happens so quickly that the first one is barely completed, or not even fully completed before the next event comes.

For example:

• Не успел я сесть, как урок начался.
  = No sooner had I sat down than the lesson began.

So the construction expresses very quick succession between two events.

The не is an essential part of this construction.

Russian uses не успел to say that there was not enough time for the first action before the second one happened. That is exactly what creates the meaning of almost immediately.

Compare the logic:

• успел = managed to, had time to
• не успел = didn’t manage to, didn’t have time to

So in:

• Не успел я открыть дверь, как зазвонил телефон.

the meaning is that the phone rang before there was really time for the first action to settle.

Without не, it would not be the same construction.

Yes, как often means how, as, or like, but in this construction it has a special function.

In не успел ... как ..., как introduces the second event: the one that happened immediately afterward. In this pattern, it is idiomatic and fixed.

So:

• Не успел он выйти, как пошёл дождь.

means:

• No sooner had he gone out than it started raining.

You should learn не успел ... как ... as a set pattern, not by trying to translate как word-for-word every time.

The dots show that this is a pattern with slots to fill.

Something goes after не успел, and then another clause goes after как.

For example:

• Не успел я проснуться, как зазвонил будильник.
• Не успели мы заказать еду, как официант принёс меню на английском.

So the structure is roughly:

• не успел / не успела / не успели + first action, как + second action

The dots are not part of the grammar itself. They just show that words are missing because this is a formula.

Most often, yes: this construction is very commonly used in the past to describe two events happening one right after another.

Typical pattern:

• first part: не успел / не успела / не успели
• second part: usually a past-tense verb, often perfective

Example:

• Не успел он закрыть окно, как снова стало жарко.

However, Russian can use related patterns in other tenses, especially in more expressive speech:

• Не успею оглянуться, как лето закончится.

That means something like Before I know it, summer will be over.

So the classic textbook pattern is usually past, but the broader idea can appear elsewhere too.

Because it agrees with действие, which is a neuter singular noun.

In the sentence:

• ... второе действие произошло почти сразу.

the subject is второе действие = the second action.

Since действие is neuter, the past-tense verb must also be neuter:

• действие произошло

Compare:

• урок начался — masculine
• встреча началась — feminine
• действие произошло — neuter

So произошло is simply normal past-tense agreement.

Because второе действие is the subject of произошло.

In:

• второе действие произошло почти сразу

the structure is:

• второе действие = subject
• произошло = verb
• почти сразу = adverbial expression

Since it is the subject, действие is in the nominative case, and второе agrees with it in gender, number, and case.

Here что means that and introduces a subordinate clause.

So:

• конструкция ... показывает, что второе действие произошло почти сразу

means:

• the construction ... shows that the second action happened almost immediately

This is a very common Russian pattern:

• Я знаю, что он дома. = I know that he is at home.
• Она сказала, что придёт позже. = She said that she would come later.

So in this sentence, что links the main statement to the explanation of what the construction expresses.

This is a heading-like introduction.

• Новая грамматическая тема: = New grammar topic:

The colon shows that what follows is the explanation of that topic.

It is similar to English:

• New grammar topic: the construction ...

This part is not doing anything unusual grammatically. It is just introducing the explanation in a textbook-style way.

It is generally neutral and very common. You can hear it in everyday speech, read it in books, and see it in explanations of grammar.

It can sound a little more expressive than a plain sequence of events, because it emphasizes how quickly the second thing happened.

Compare:

• Я открыл дверь, и телефон зазвонил.
  = I opened the door, and the phone rang.

• Не успел я открыть дверь, как зазвонил телефон.
  = No sooner had I opened the door than the phone rang.

The second version is more vivid and strongly highlights the immediacy.

No. The subjects can be the same or different.

Same subject:

• Не успел я поесть, как снова захотел кофе.

Different subjects:

• Не успел я войти, как дети закричали.

What matters is not who does the actions, but that the second event happens almost immediately after the first one begins or occurs.

Very often, the second action is expressed with a perfective past verb, because Russian is presenting it as a completed event that suddenly happened.

Examples:

• Не успел я сесть, как поезд тронулся.
• Не успела она открыть книгу, как свет погас.

In the first part, after не успел, Russian often uses an infinitive or a short action that sets up the next event.

The key idea is:

• first event: just happened / barely happened
• second event: followed immediately

So learners will often see compact, event-like verbs in both parts.