Когда учитель вошёл в класс, студенты замолчали.

Because this is motion into a place.

• в класс = into the classroom
• After в:
  • accusative is used for destination / motion toward
  • prepositional is used for location

Compare:

• в класс = into the classroom
• в классе = in the classroom

So учитель вошёл в класс means the teacher went into the classroom, not that he was already there.

This is a question of aspect.

• войти / войти́ and its past form вошёл are perfective
• входить is imperfective

Here, Russian uses вошёл because the sentence refers to a completed event: the teacher entered the classroom.

Very roughly:

• Когда учитель вошёл в класс... = When the teacher entered the classroom...
• Когда учитель входил в класс... would sound more like When the teacher was entering the classroom..., focusing on the process rather than the completed entry

In this sentence, the completed action is what triggers the students’ reaction, so вошёл is the natural choice.

Because замолчали means became silent / fell silent, while молчали means were silent / were keeping quiet.

• молчать = to be silent, to keep silent
• замолчать = to fall silent, to become silent

So:

• студенты молчали = the students were silent
• студенты замолчали = the students went silent

In this sentence, the point is that the teacher entered, and then the students changed state. That is exactly what замолчали expresses.

Here, за- often gives the meaning to begin doing something or to enter a new state.

So:

• молчать = to be silent
• замолчать = to start being silent, to fall silent

This use of за- is very common in Russian:

• заплакать = to start crying
• засмеяться = to burst out laughing / start laughing
• заговорить = to start speaking

So замолчали is not just were silent; it means they went quiet.

Because Когда учитель вошёл в класс is a subordinate clause introduced by когда.

Russian normally separates this kind of clause with a comma:

• Когда учитель вошёл в класс, студенты замолчали.

The same thing happens in English:

• When the teacher entered the classroom, the students fell silent.

If you reverse the order, Russian still uses a comma:

• Студенты замолчали, когда учитель вошёл в класс.

Those are past tense endings.

In Russian, the past tense is formed with -л plus endings that show gender in the singular and number in the plural.

For войти:

• вошёл = masculine singular
• вошла = feminine singular
• вошло = neuter singular
• вошли = plural

For замолчать:

• замолчал = masculine singular
• замолчала = feminine singular
• замолчало = neuter singular
• замолчали = plural

So:

• учитель вошёл because учитель is singular masculine
• студенты замолчали because студенты is plural

In this sentence, yes: it strongly suggests a male teacher.

That is because the past tense verb is masculine singular:

• учитель вошёл = a male teacher entered

If it were clearly female, you would normally expect:

• учительница вошла в класс or
• учитель вошла в класс in contexts where учитель is used as a professional title for a woman

So the form вошёл points to a male subject here.

Russian has no articles.

So учитель can mean:

• a teacher
• the teacher

And в класс can mean:

• into a classroom
• into the classroom

Russian leaves this to context.

In this sentence, English usually says the teacher and the classroom, because the situation sounds specific and definite. But Russian does not need separate words for that.

Yes. Russian word order is more flexible than English word order.

You can also say:

• Студенты замолчали, когда учитель вошёл в класс.

That means essentially the same thing.

Why use one order or the other?

• Когда учитель вошёл в класс, студенты замолчали.
  This sets up the time/background first.
• Студенты замолчали, когда учитель вошёл в класс.
  This puts the main event first.

So the original sentence is natural because it first gives the condition or moment, then tells what happened.

The important point is that ё is always stressed.

So вошёл is pronounced roughly like va-SHYOL or va-SHOL, with the stress on the last syllable.

A few useful things to know:

• ё is never unstressed
• In many Russian texts, ё is often written as е, so you may see вошел
• But it is still pronounced вошёл

This matters because stress in Russian is very important, and ё helps you know exactly where the stress goes.

Когда introduces a time clause.

Here it means when:

• Когда учитель вошёл в класс = When the teacher entered the classroom

The whole sentence structure is:

• Когда + event A, event B

So literally:

• When the teacher entered the classroom, the students fell silent.

Russian uses когда very much like English when in this kind of sentence.

Yes, both are perfective:

• вошёл from войти
• замолчали from замолчать

That makes sense because the sentence describes two single completed events:

1. the teacher entered
2. the students fell silent

Perfective verbs are often used in narration when one completed action leads to another. That is exactly what is happening here: the teacher’s entrance is a finished event, and the students’ silence is another finished event that follows it.

If imperfective forms were used, the sentence would describe the situation differently, usually with more focus on an ongoing process rather than a completed sequence.